I'm with NTL, you get a nice cable modem with both ports on. IIRC, most cable modems are actually designed for Ethernet but are shipped with USB adapters.
Why? Supposedly to simplify the installation process for the less technical PC users. It's a really bad compromise to make though, connecting through USB on my PIII-600 slows the whole machine down. And it takes up a useful USB port! :-D
It's a shame they have to keep their users in the dark about this... I wonder how many people feel like they're wading through treacle trying to use their machine, not even aware they can use Ethernet.
