I definitely think that the shift away from Microsoft will be a gradual one starting with people using OSS programs in MS Windows. Firefox/Mozilla have already made great progress in this arena, and I hope soon OpenOffice will begin to supplant MS Office as well. Once the "everyday user" gets used to using OSS applications, switching the underlying OS won't be such a big deal. One of the biggest complaints I get from new Linux converts is "Where is my [insert MS product]? I need to [insert random easy task]."
I'm not a prophet, but I'm going to throw out some numbers just for fun. I'd say OpenOffice (provided it isn't sued into oblivion, MS has done it before) will have half the productivity users in another 5 years, development tools like the Eclipse IDE will replace Visual Studio and .NET in another 10, and the Linux OS will be about 50% of the desktop market in about 15 years. These are of course based on absolutely nothing.
