Yep, I've got this on my ST as well.
Since I never, ever, never, ever use the back brake I don't even bother bleeding it until MOT time. The MOT was last week and now my back brake is sticking on.
Could be that I've overfilled the reservoir when bleeding the brake or it could be that, since I never use it, the pistons are just stuck. Either way I don't think it's anything that a good clean and grease up can't fix.