Depends on the modem, but yes, most DOCSIS (nearly all) compatable cable modems are "uncappable" for the most part.
Pretty much lazy cable modem firmware coders, which allow you to essentially poison the modem's arp cache and have a server on your network "become" the tftp server the CM downloads it's config file from (which is what contains the speed cap settings, among other things).
No, I won't help you. No, there are no (that I know of) scripts or tools out there that easily let you do it (yet).
Fair warning. A friend of mine (who actually figured this stuff out) did this and get caught at it. It's fairly easy to detect if you do any sort of uploading at all. I mean, when they see traffic spikes 10X what they should be for your upstream service class... Granted, they probably won't notice normal usage though.
If you truly are interested, look at
http://online.securityfocus.com/archive/82/261280
-Phil