There seems to be a lot of pain out there right now over missing default gateways when Windows 2008 SP2 is applied.
A hotfix is not available yet at the time of writing.
For now you have one of two options
a) don’t use SP2
b) if you need to use SP2 backup the DefaultGateway and DefaultGatewayMetric registry keys from HKLM/System/CurrentControlSet/Services/Tcpip/Parameters/Interfaces/%CLSID% and re-apply them after rebooting from SP2.
If you are installing SP2 remotely then you WANT to reapply these values as part of a MACHINE startup script, or you will loose connectivity to the machine.
Vista SP produces identical results at this stage, so the same fix applies.