@Jason: VSS is broken.
I think the most powerful method for motivating a change away from VSS is to point out how critical an asset your source code is. Taking risks with its integrity is not a wise business choice.
Add that your programmers are the creators of this asset, and that making it easier for them to be productive means more value in your source code asset. Joel on Software often talks about how investing in his programmers is a big win for his company.
The other answers here all describe specific reasons that you can point to when making your case.
