I just ordered a rock tumbler to do this. I need a voice of experience: what size grit? How long?
Well, I answered my own question through WikkiPedia: no grit (just water), about 4 hours is enough to remove those sharp edges. I'm using a $25 Smithsonian tumbler from WalMart.com. It holds about 1/2 wine bottle at a time.
I have been tumbling broken bottles to use as a type of mulch around stained glass stepping stones in my garden and have found that a cement mixer works great so that you can do several bottles at a time....saves a lot of time and hassle.