Then check out this KB article, seems like this is hitting quite a few folks who are deploying SP2. Could be older network drivers, could be firewall type software, could be other advanced network software.
You cannot host TCP connections when Receive Side Scaling is enabled in Windows Server 2003 with Service Pack 2
There is also some interesting info in the SBS groups and blogs as this really seems to have smacked them pretty well.
Here is a good link
joe
Per susan about 10 minutes ago there is now an SBS patch that does something to mitigate this on SBS available on Windows Update. they have it under optional updates for some odd reason though