I'm ridiculously excited too, and I'm old enough to remember The Five Doctors... eeek!

However I'm going to assume the only old Doctor to appear will be Tennant, since Eccleston has turned it down (gutted), and McCoy has gone on record as saying none of the older Doctors are appearing...
Even so I think it will be brilliant, and maybe not having too many Doctors appearing might help the story, in that there won't be loads of old actors with nothing to do!