It might be worth digging a bit deeper into the 'contract' The reason why they dont like you to use Linux is becaus Linux tends to use Masquerading rather than the Proxy that is common with W$.
NO, accoring to the Telewest Blueyonder installer (who was a cool techy not just a jobsworth) that installed mine its because Microsoft own 26% of NTL and therefore just "don't support" Linux.
http://www.xciv.org/byhsi/ is a handy link too
I think the sales rep said they currently recommend "Terayon TeraJet 21=
0"
cable modems.
NTL provide you with a Motorola Surfboard cable modem - this is a good thing if you're technically minded becuase there are plenty of resources on the net to tell you how to get a little more than you're paying for out of your modem (I know a 15 year old kid in Essex on NTL getting 2GB) but of course this is not something I would condone ;)
I myself have Telewest Blueyonder connected to an old piece of junk 133MHz Intel running as a smoothwall router www.smoothwall.org - I let the installer set up on a pc with windows on it then transferred the NIC to the crappy box and installed smoothwall to maintain the MAC address.
Best regards,
Dave