So I'm at the in-laws this week, and they've got a PC with cable internet. I tried unplugging the cable from the tower and hooking it into my macbook, but no luck.
I don't know how cable works, but I'm assuming that it's similar to DSL. Is there an Ethernet cable from the modem to the computer? If your Mac is set to obtain addresses automatically (DHCP) then it should work without any problems. If it's USB then you may need to use some sort of connection software (PPPoA?). I'm sure someone else will be able to help a bit more