There are a few ports of rails migrations to the .NET world (that's where I spend alot of my time). I have not used any of these for 'real' but I have 'sampled' them. They all look promising but it seems like there is not very much activity on these projects.
I don't know if they ever did incorporate this into the official release of sub sonic? maybe it will come in a future release
I recently stumbled across vincent-vega and have begun using it on a small project and am planning to incorporate it into some of our larger applications. It is an Nant task that handles database versioning. It is not a migration in the same way as the other solutions as it uses actual sql scripts for all schema changes instead of c# code, which makes sense to me!
vincent-vega is now part of the tarantino project.
Thanks for the heads up Dan!