Even simpler: it's the T-shaped BNC coax adapter you use to connect a PC to the coax network. If you don't have a BNC/coax input on your PC any more, you could use a simple (older?) SOHO office hub that features both, BNC and RJ45 connectors, plug the T into BNC and your PC into one of the RJ45 connectors.
Hacking the firmware on your old Motorola Cablemodem is the best bet.
~Mike. _______________________________________________ Full-Disclosure - We believe in it. Charter: http://lists.grok.org.uk/full-disclosure-charter.html Hosted and sponsored by Secunia - http://secunia.com/