Those four letters are the shortened version of Family Active Sports Tourer. We'd prefer to call it a wagon, but FAST (the model's internal designation according to AutoExpress), is far better than BMW 1 Series Family Active Sports Tourer Fdrive30iT... or whatever the company's marketers would prefer to call it.
Speculation is that the BMW FAST will take over for the 1 Series, which itself will soon graduate to become 2 Series. This new wagon will utilize a front-wheel-drive platform that's shared with the next crop of Mini vehicles. Rumor has it that an all-wheel-drive version will also be made available. We're not counting on it keeping the FAST designation when it reaches showrooms, but either way, look for this creature to arrive by 2013.