I originally posted this little titbit in the wcf forums, but since lots of people there did not seem to know about it, I'll post it here again:
The framework 3.0 RC1 install will kill your existing .svc mapping, so you will not be able to communicate with IIS hosted WCF services (.svc). Obvious workaround is to add the mapping again manually. This will be fixed in the next RC.

I hope that helps a few people out there!