In a blog post back in September of last year I described how to use a custom task to determine whether tests had passed in Team Build v1. In Team Build Orcas, you should be able to accomplish this same goal using the new built-in task GetBuildProperties. The following XML snippet in your build script (TfsBuild.proj) should do the trick:
<GetBuildProperties TeamFoundationServerUrl="$(TeamFoundationServerUrl)" BuildUri="$(BuildUri)">
<Output TaskParameter="TestSuccess" PropertyName="TestSuccess" />
The property TestSuccess would then be 'true' if and only if all test runs succeeded for the build, just as in my previous post. A bit easier, no?