No Tom's not going to appear in the TV show in some kind of Time Crash event as he told Doctor Who Magazine he would not want to do that. Instead he is starring in a new series of Doctor Who audios, called Hornet's Nest, featuring his doctor post-Invasion of Time, travelling to 2009 and renting a cottage on the south coast of England and getting into all sorts of whacky adventures.
These are made by the BBC, not Big Finish, which is a little bit of a kick in the teeth to that company, but sadly Tom Baker seems to do things his own way, rather than associating with anything involving the other doctors. His reason for doing these audios is (very Tom Bakerish) that he was sick of saying no to the various reprisal offers he gets, and because he wanted to work with Nicholas Courtney (or more likely go down the pub with him after recording).
Interestingly his companion is Mike Yates, a Pertwee-era companion. Sadly it was originally intended to be a very old Brigadier, but poor Nicholas Courtney is not very well. I hope he has a speedy recovery and gets involved in a 2nd series of Tom Baker audios. I also hope people's bank balance can afford all these radio plays from Big Finish and the BBC.
A dedicated website is being setup by the BBC and it looks like just the thing before the next season of McGann audios start up in December again!