So far since updating to SP1 on my Windows 7 Pro x64 machine a few days ago and updating to Avast 6 I have had issues with several firewalls. PC Tools Firewall 7 had either an issue with Avast or SP1 or likely both, could not connect over TCP, web browsing was dead, downloads would freeze. It worked fine pre SP1.
I tried Privatefirewall(the latest version), it would connect fine but for some reason on reboot I could not click on anything in it's GUI to make changes.
I tried for the hell of it even though I hate it Comodo 5.3(whatever the latest version is)and I could not connect over TCP with the Avast web shield enabled. Web browsing was dead. I had all of Comodo's defense garbage disabled.
Now i'm using Online Armor Free 4.5.1.431 and knock on wood nothing is F'd up...yet. I disabled all of it's HIPS features, don't need it, straight firewall. By the way I usually test software in a VM but currently I only have XP loaded up in one which isn't a good way of testing issues with software within Windows 7 obviously. So I would try a firewall in Windows 7, and then restore to a clean image if it didn't work. So it wasn't like I had residue files hanging out from other firewalls screwing things up.
So in the end, it seems Avast 6's web shield is having issues with certain firewalls or vice versa. Some other firewalls are having issues with Windows 7 SP1.