Before I sold my original modem (I think it was a Surfboard 4800), I successfully uncapped my modem from 30kbps all the way up to 256kbps. It was awesome. My ISP is a small, po-dunk cable ISP, and I ran uncapped for two years, before I moved away to college, and sold my modem to a classmate since I had DSL at my apartment.
How I wish I still had that modem.
The 5100 however, seems pretty well locked down. :-l