If you missed it, here's the 8-minute Doctor Who special shown on Children In Need last night, written by Steven Moffat. For me, it was particularly amusing because Peter Davison was the Doctor I grew up with as a kid, so watching Doctor #5 interact with David Tennant's #10 was very entertaining.
It was also particularly wonderful how Moffat managed to answer a few niggling questions (why the TARDIS interior changes), prevent continuity issues (why Davison's Doctor looks older), throws in some in-jokes (LINDA, The Master, etc.) and make everything sit neatly as a "missing scene" in the season 3 finale's closing moments.
Great stuff!