I would assume that throughout this ordeal you have powered down your cable modem several times, but thinking back to my own days with cable broadband (sigh... oh happy days) I remember having to power down the cable modem, wait 30 seconds, power it up, wait till all lights were steady green, then powering down and restarting the modem again before it would accept a new MAC address from a different source (like when switching from a directly-connected PC to a router). Just to make sure though, if you haven't already done this, I would run through that procedure.