Aaron Hallberg has a great post and downloadable target file that describes how to pass your custom properties down to solutions that you are building. In Version 1 of Team Build, there was a disconnect between the properties that you set in your TFSBuild.proj file and those that actually got to the MSBuild instance that compiled your code. This override solves that problem by utilizing Item Metadata on the SolutionToBuild item group. This is an elegant solution to this problem and goes to show just how configurable and extendable Team Build is.
Kudos to Aaron and the whole Team Build team for listening to us and providing additional value out-of-band.