Unfortunately, the sixth Doctor, Colin Baker, denied any knowledge of a multi-Doctor episode on Twitter. He was accused of just "hiding the truth", but he quickly quelled that assumption, and indicated that no one has approached him to reprise his role.
Moffat mentions in the tail-end of this Youtube interview the pressure of not wanting to let anyone down for the big anniversary episode, so even if it is not a multi-Doctor escapade, we can expect something to delight all Whovians.
If the rumor were true, this would not be the first time multiple Doctors have had to work together. The 10th, 20th, and 30th anniversaries all featured crossovers with different regenerations. And, an episode with all 11 would be one awesome episode! I would most look forward to Eccleston reprising his role...he was my very first Doctor, and probably my favorite.
However, I have to believe that it's probably not true. Just think of cramming 11 different Doctor personalities into one episode...it would just turn into one giant ego war, and I can't imagine anything getting accomplished at the end of the episode (except leaving fans with a lot of unanswered questions!)
Do you have anything you're looking forward to in the 50th Anniversary Special?