In reply to The Lemming:
I've done both, and find I have much more control via stacking, but that sinking feeling that you're not doing it all inside the camera and are therefore cheating in some way. What I mean by more control, is that if you take a bunch of 30 second exposures and merge them in photoshop, any light pollution has been reduced massively, as you've only exposed for 30 secs instead of potentially hours. Also, it limits your susceptibility to a single catastrophic event that reduces your entire nights work to nill, like a passing mountain biker with a headlight so powerful you forget its night. Another good advert for stacking is that it’s much easier to clone out the flashing lights of passing aircraft for instance, as your image will be less busy for that 30 second exposure. I would also take one image specifically exposed for the foreground, cut it out in photoshop and stick it on the front of your stacked images. I suppose the main pro of the long exposure alternative, is that there’s less post processing.