Update, October 2011: The Colosseum has just announced that the underground and third level will remain open through… Dec. 31!
After weeks of wondering whether the Colosseum underground and third level would be open in October, good news: They are. Colosseum management Pierreci just confirmed the hypogeum and third tier opening through the month.
As before, these areas are open only to those on specified tours, and must be booked in advance. (The official, Colosseum-run tour, which is one of several options that I explain in the next link, is about an hour long. It includes only these new areas, although the guide will of course be talking about the Colosseum in general, and you'll be left in the Colosseum itself afterwards to explore the rest on your own). Here is more information on the different tours available of the Colosseum underground; here are some photos of what to expect of the Colosseum's underground and top tier.
I'll be updating this post with more information as it comes in—such as whether the areas will be open in November—so keep checking back here.