Good news for those hoping to dodge the line at the Colosseum and Roman forum: Now, you can buy skip-the-line tickets directly from Pierreci, in advance.
Before, your options for skipping the line at the Colosseum were as follows. First, you could go to the ticket desk at the forum or Palatine hill to get your combined Colosseum/forum/Palatine ticket; although this usually worked, it wasn't always foolproof (when friends visited this spring, we went to the Palatine window at noon and still had to wait for a half an hour). Second, could go through a third-party ticket-sales service online, which added a hefty booking fee. Or, third, you could book a tour with a reputable tour company (still something I recommend, since fully appreciating the scandalous, salacious stories behind Rome's ancient ruins can be tough on your own).
But now? It's even easier. Just go to Pierreci's new ticket booking portal for the Colosseum, scroll down to where it says "Purchase your ticket," and choose your option—online or (whooooa) by iPhone. (For the iPhone option, you have to download Pierreci's app).
Once you've reserved, you can either jot down the code and bring it to the reservation desk, or print your ticket off yourself. Either way, you don't have to stand in that absurd line wrapping around Rome's most depressing most popular sight.
It's €1.50 extra, per person, to reserve your ticket. The ticket itself is the same €12 combo job (forum, Palatine and Colosseum) that they've had for a couple of years now.
As an aside, Pierreci's new website is really, really slick. Is this a sign that Rome's tourism is emerging from the Dark Ages?
I've gotten a lot of messages asking more about how to access the subterranean and third levels of the Colosseum, which officially opened to the public today.
Well, I've been there, done that (I was actually lucky enough to be on the very first public tour of the newly-unveiled areas at 9:40am this morning!), so I'm happy to share!
Update, July 2011: It's been confirmed that the Colosseum's underground is now open through September… but possibly no later! Click the link for info on the three major (and only) ways to get to the Colosseum underground. (The post below gives you info on how to book by taking a tour with the Colosseum directly — but that's not necessarily, or always, the best way).
Update, March 2011: The Colosseum's underground has reopened! Some things, including the exact price and the ability to pay in cash on the day of, have changed. Click the link for more info.
Why is this special? It's the first time since antiquity that the hypogeum and third levels have been officially, safely open to the public. (Actually, even better than that, since even in antiquity the hypogeum would not have been open to the public). And it's the first time the arena has been open to the public during the daytime.
Do I have to book in advance to see the hypogeum and third levels? Yes, you must book in advance.
But why? Because the areas are archaeologically sensitive, they don't want the Colosseum's 19,000 or so daily visitors clambering around on their own. Instead, the Colosseum's official guides are taking groups to those areas, with a maximum of 25 people per group.
How do I book? The best (and, I think, so far only) way to book is to call Rome's cultural association, Pierreci, directly. Their phone number is +39 06 39967700. Websites might start cropping up soon, if they haven't already, offering to sell you these tickets online; they'll charge you a surcharge for this, so just call Pierreci instead.
Do they speak English? Yes, they should. If you can't follow the rapid-fire Italian for your options through the automated system, press 0 the first time you're asked a question, 3 the second time. (Assuming, of course, you're an individual booking for a group tour). That should bring you to an operator. Once you speak to someone, if you just ask, "Parla inglese?", you should be able to communicate with them in English fine. They are, after all, offering English guided tours!
But I don't want to make an international call. That's expensive. Download Skype (www.skype.com). It's a free voice-over-internet program and takes thirty seconds to download. It's intuitive, it's easy, and you can call phones internationally for much less than what most phone cards would cost you. Plus, calling Skype to Skype is free.
How much is it? It costs €12 (the normal entrance price, which includes your entrance to the Forum and Palatine), plus €8 for the guided tour, plus a €1.50 reservation fee. You do not pay in advance.
What times are the tours? I don't know, and I'm not sure there's a regular schedule. But it seems like lots of tours in both Italian and English (perhaps other languages, too) are being given. The operator will give you a list of times that you can choose.
Should I book now, or wait till I get to Rome and have more of an idea of my schedule? Book now. Seriously, everyone and their mother will want to do this. You need to get a slot as soon as possible.
How long are they doing this for? So far, till November 30. But I can't imagine they won't continue it after that.
So I have my reservation number, and I'm in Rome. Now what? When you go to the Colosseum, you'll see a long line on your right. Don't stand in it–that's for people without reservations. You also might see a long line on your left. Don't stand in that one, either, which is for big group tours. Instead, go down the middle. When a guard asks you for your ticket, say you have a reservation. He'll let you through to the ticket windows at the end. Get in the line for reservations ("prenotazioni"), which should be very, very short. At the window, present your reservation code. You're then given your ticket, plus a little sticker saying you're one of the chosen few for the tour. The meeting point is currently in front of the elevator, but that may change (like everything does!), so make sure to ask.
What if I have a RomaPass? If you tell the person at the reservation window that you already have a ticket to the Colosseum, you can pay just the reservation and tour fee and use your pass.
Can I use my ticket for something else? Yes. It's a normal, combined Colosseum ticket, so you can use it for entrance to the Forum and Palatine for the rest of the day and the following day (the deadline will be printed on your ticket).
What does the tour cover? The tour includes the hypogeum (subterranean area), arena, Porta Libitina, and third level.
How long is the tour? This morning, it took us an hour and a half to get through it all. I don't know if that's how long they'll all be.
Can I sightsee more around the Colosseum after the tour? The tour ends inside, so yes.
Is it worth it? Yes. It's incredible. And this is absolutely how the Colosseum is meant to be seen. Once you see the Colosseum from its almost-top, and peer up at the seating from where gladiators and animals would have waited for their turn, and walk through the gate where gladiators' dead bodies were taken out, you'll feel badly for those who "only" got to see the Colosseum's first and second levels.
Whew! I hope that covers everything, but let me know if I left anything out!
A guard allowing us up to the third level of the Colosseum.
Good news for those hoping to dodge the line at the Colosseum and Roman forum: Now, you can buy skip-the-line tickets directly from Pierreci, in advance. 
