In 2010 they opened what is called an subterranean tour. Besides the underground part, It actually includes visiting the top of the Colosseum that is not normally open to tourists. It is a different ticket than normal, limited in volume and subject to weather. ABC News has a good article on the tour here.
As we had been through the Colosseum before my wife didn't want to go back. Previously though we hadn't been through the Forum as it is so rambling we didn't know what we were seeing. I finally negotiated with her that she would go on the Colosseum underground tour if we could get a tour guide for the Forum. We booked a tour with Tickitaly that was wonderful. It cost 67€ for both of us.
By the way the meeting place is right here but the tour guide was not easy to spot.
Incidentally Tickitaly has a good article on the Colosseum here.
The wikipedia article is here.
We walked from our hotel to the Colosseum that morning as we had heard that there was a transportation strike planned for that date. Interestingly the Roman transportation strikes start after everybody gets to work and are over before they get off so as to not disturb the workers. Only in Rome! There's a subway map here.
View Larger Map