Aaron pointed out this post by Ben Day that talks about using the RunConfigFile property with a build that runs tests without using test lists (.vsmdi files).  It all boils down to looking like the following (in Orcas, you would specify the $(OutDir) part in the TestContainer's Include attribute, whereas you don't in the TFS 2005 power tools TestToolsTask TestContainerInOutput element).

  <ItemGroup>

    <TestContainer Include="$(OutDir)\HelloWorldTest.dll" />

  </ItemGroup>

  <PropertyGroup>

    <RunConfigFile>$(SolutionRoot)\CSharp\Tests\localtestrun.testrunconfig</RunConfigFile>

  </PropertyGroup>

Read Ben Day's post, Run tests from your TFS Team Build without a test list (.vsmdi) with code coverage, for the details on setting this up with the TFS 2005 power tools task.