This morning Twitter went crazy as fans reacted to new images that appear to show Tom Baker back in the scarf as the Fourth Doctor. Filming is currently taking place in London.
If this is something official, then it is likely that the BBC will announce the details soon, as they did when filming the anniversary special with David Tennant. Otherwise, we may be looking at another long wait to see what's really going on.
UPDATE: The filming is a promotion for Have I Got News For You, and it is Paul Merton who is dressed as the Fourth Doctor, and Ian Hislop as the Seventh Doctor.