I can't speak for ISDN because I've never used one.
As for Cable Modems, I have a COM21 modem with service through AT&T @Home and it works great with Linux. It connects to a standard 10 or 10/100 Ethernet card. Initially I had trouble with DHCPbut that was due to signal strength not any hardware issue. This was no big deal because they set me up with a static address while they resolved the issue. Once AT&T fixed the problem DHCP worked fine. Best part is no special configuration, justsetup the network like normal and away you go.
As for USB, it will be supported starting in the 2.4 kernel, which should be released soon. They just released a test version last night so it's really close. However, you will still need to find drivers for your USB devices and that may take a little longer as the drivers need to be written. For common devices, like mice, you should be able to find drivers immediately. For less common devices, like scanners or cameras, you may have to wait a while or write them