I’ve seen a lot of questions recently about various permutations of building VS 2008 apps with a TFS 2010 build service, building a VS 2010 app with a TFS 2008 build service, etc.
Jim Lamb wrote a great FAQ post that I highly recommend you read. The only thing he didn’t cover is building VS 2008 apps with a TFS 2010 build service – because it should just work. The only thing you need to do is to install VS 2008 on the build server because VS 2010 doesn’t include all of the compiler/language components necessary to build VS 2008 apps. But once you’ve done that, you should be able to just check in your 2008 solution, create the build definition (with 2008 or 2010) and start the build – it should work.
Update: Madhuri shared with me a post she wrote about things to look out for when building VS 2008 projects with TFS Build 2010: http://blogs.msdn.com/madhurig/archive/2009/11/25/building-vs2008-projects-with-tfs-build-2010.aspx