There is a new transcript online from the Sweeney Todd press junket which Alan Rickman attended. In the interview he confirms he will be back as Professor Snape.

QUESTION: How did your work schedule play into the next ‘Harry Potter’ film? Did you go from one to the other or did they overlap at all? I know that in ‘Half-Blood Prince’ there’s a lot for you to do in that film?

RICKMAN: Yeah, but they still have to get inside a certain period of time. I do seven weeks. I can’t even remember when we shot the last one so I think – we shot ‘Sweeney Todd’ this year – it was over a year ago that I did ‘Order of the Phoenix’.