Well, it could be the problem. When I first started with Comcast, they required me to give them the MAC address of my modem. If I didn't give it to them, or changed the modem, I couldn't connect. Then, about a year ago, another friend signed on to Comcast, and it DIDN'T require the MAC address. Now it appears they're back to requiring it. Part of the reason I've moved from Comcast to Verizon (including TV), they don't seem to stay consistant with anything.
Another possibility (and more likely cause) is that they wiped out their authentication database, where they store all the MAC addresses who are registered - so you may find a few of your neighbors running into the same problem, if that's the case.