It's a Comcast problem, if you have Comcast there's a good chance your Steam is borked.
You have three options, according to some dude on the Penny Arcade forums:
1) Wait for your DHCP lease to expire (which usually takes several days)
2) Call Comcast, explain the problem, and ask them to give you a new DHCP lease with a different IP address.
3) If you have a router that can clone your PC's MAC address, you can use this to trick Comcast into giving you a new IP address.