Morning Spoilers posts hints on The Lodger, quoting
SFX.
A certain face shows up for the fourth time this series; another for a third; and others for a second time
As the io9 writer comments, that's almost certainly the preceding 10 Doctors; we've seen Hartnell three times, Troughton twice, and the rest once.
While I think the io9 guy's theory that it's building toward a Four Doctors crossover is farfetched (and, in fairness, he seems to think so too), it DOES seem like the previous incarnations bit has been repeated often enough that it's gone from being a fun bit of fan service to Probably Important. Plus, we've got River Song knowing all the Doctor's past selves, the Doctor fighting himself, a piece of the TARDIS on the other side of the crack, and River being all set to kill him -- yeah, I think regeneration/legacy/identity/self are going to be very important themes in the finale.
(And while I think it's still too early in Smith's run for a Big Doctor Team-Up Story -- ESPECIALLY one with Tennant, who hasn't been out of the role for SIX MONTHS yet -- this COULD all potentially lay groundwork for one a few years down the road. Fiftieth anniversary is in 2013...)