So, yes, Linux has full ATM support, which means it also works fine with many popular USB ADSL routers, including: Speedtouch, Conexant AccessRunner, ADI 930 & Eagle, and others which share the same chipsets as those listed.
EDIT: Note that some of these (eg. AccessRunner) require downloadable firmware -- rightly not part of the Linux kernel -- and this all works fine with the in-tree kernel drivers.
The best solution for stuff like this, is to acquire hardware that has an ethernet interface on the downstream side (rather than only a USB interface).
Nice, open IP/PPP protocols there, and no drivers required for any kind of downstream equipment. Plus the
option of having the device act as a small LAN gateway DHCP/DNS server, independent of any specific machine. That's not how I use mine, but many people do use them like that.
Cheers