While the cable companies that are selling "Cable Modem" services claim only to support Windows 95, all of the cable modems I'm aware of require an Ethernet card in the PC, and thus should be able to be attached to a Unix LAN using regular Ethernet connections.
Note that calling the devices "Cable Modems" is ridiculously inaccurate. These devices are in fact not modems at all, as they neither modulate nor demodulate between digital and analog signals. In similar fashion, there is no such thing as an ISDN "modem." The whole point to ISDN is to not need the digital/analog conversions that a modem is intended to provide. The typical correct name is to call it an "ISDN Terminal Adaptor." Consider this yet another conquest of marketing idiocy over reality.
Check out the following URLs for more info:
For those in North America that spend significant time on the road, it is important to have some sort of "distributed access" to the Internet. There are service providers like MCI/AOL/Compuserve that are large enough to have national presences; there are also groups of ISPs that have joined up together from different regions so that you can so something like the following:
Call some central (tollfree) number to determine where to dial in in the local area
Provide a tollfree service that provides direct connections, perhaps at additional cost...
Provide common authentication so that you can connect at various points of presence with a single ID.
I might also register a domain through Dotster, Inc for this...