Back in June, I wrote about the upgrade of the main Developer Division server to TFS 2012 RC. That post described both the scale of that server as well as the issues that we found in the process of upgrading it and the for the first week of running it (we found most all of the issues in the first week).
Jason Prickett has written a blog post summarizing issues and workarounds for problems encountered with TFS build when upgrading from TFS 2010 to TFS 2012.
Upgrading your build definitions from TFS2010 to TFS2012
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[Update 11/26/12] You can get the fix by installing Update 1 (or newer) for Visual Studio 2012: http://www.microsoft.com/visualstudio/eng/downloads.
Some customers, after starting to use Visual Studio 2012 with their existing TFS deployment, have been receiving check-in policy errors having to do with the Changeset Comments policy.