Richard Berg (BUGBUG) from the Developer & lifecycle tools at Microsoft says:
If you plan to use VS 2008 with SourceSafe, make sure to pick up the Update CTP too. Without it, some features like "Open from Source Control" will not work at all.
We are working on an update for Visual SourceSafe 2005 to make it work with VS 2008. We had originally planned to have it available at the same time as VS 2008 downloads went live but we hit a last minute bug that is taking a little time to work out. Our current expectation is that it will be available in mid December.
Not that I'm using SourceSafe any longer (if I can avoid it...) but I'm sure lots of people have already installed VS2008 and wondered about VSS support.