Chris Lough at Tor.com has done a lovely breakdown of lots of things we can see in last week's trailer. Not least of which appears to be the Tenth Doctor on a horse. It probably isn't David Tennant, because we know they didn't let him ride the horse for real in Girl in the Fireplace (could this be the same horse, story-wise?). But the suit is the right colour, and the TARDIS interior matches, too.
Anyway, have a read of the other observations at http://www.tor.com/blogs/2013/11/doctor-who-50th-anniversary-trailer-hints.