A word of warning: between forged IPs, compromised systems, and formerly hostile IP space given to new owners, an IP blacklist will eventually hit legitimate customers. I speak from experience on this since I had the same bright idea.
You are right about the blacklist, however, it's very unlikely you will have legitimate users coming from datacenter IPs. I've used this trick to prune hundreds/thousands of bad accounts in a couple of forums. You need to be careful with it, but I think it's a worthwhile method.
