I would try some compressed air (DustOff kind perhaps). They come with a small plastic tube which should be able to get in there. Try pressing down on an adjacent key to expose the underside of the dirty one.
The keys are snapped on, but I personally wouldn't deliberately pop them off unless absolutely necessary. There's a risk of breaking either the key cap or the base part. I'm assuming the MBP keys are the same as PB G4's - I haven't had one pop off the MBP yet to tell for sure.