It wasn't a cliffhanger, though. I wonder if this means that there won't be another season.
Update: One of the BBC digital channels just aired a making-of documentary on season 3; for the most part, it was an excuse to recut highlights from the past season with Franz Ferdinand and the Scissor Sisters in the background, interspersed with interviews with cast members talking about how they see their characters and actual former MI5 agents (including David Shayler) talking about how inaccurate the depiction of their former occupations is. According to it, season 4 is being filmed now.
the bbc website for spooks confirms there'll be a season 4 next year.