I recently had a need to create an MSBuild file for a complex (multi-project) solution in VS2010. By default .sln files are not in msbuild format, but they can be called from MSBuild - while I could have done this, I didn't want to - I wanted a proper MSBuild solution. Fortunately, there is a way to convert the .sln file into an MSBuild .proj file. Open up a Visual Studio command prompt and type in the following:
msbuild <<your project>>.sln
That's it. If your solution builds successfully, an addition MSBuild .proj file is created (this is the file that MSBuild creates internally to process .sln files). It's that simple.