Re-watching Flesh and Stone this morning I was suddenly struck with a bizarre thought.
Ok, the crack in time is obviously this seasons theme and running storyline, and it is rewriting history.

What if the Moff is actually planning for more than this season? What if he's thinking ahead to 2013?!!

If he wanted to write a series of episodes or whole season to celebrate the 50th anniversary and wanted to use the surviving previous Doctors he could do so very easily now.

They could have regenerated at different points in their timestreams, thus explaining their aged appearances.

Just an idea as I said. But possible....

Ok. Let the flaming commence!

I got as far as wondering why the master of the non-linear narrative didn't do more with a device that allows him not just to have different people interacting from different parts of the same story but to have people in the same part of the story having reached there from different versions of earlier events. The vague reference to things working differently for time travellers does seem to suggest that the sort of multi-Doctor thing you suggest could be possible, but I remain as unconvinced as ever that they'll do it; bringing back multiple Doctors is too messy a narrative.