In an earlier post I described how one can, in Orcas, preserve the output directory structure used in a standard IDE or desktop build.  It seems that many people are looking for a simple approach, however, to augmenting the standard Team Build output directory by putting the outputs of individual solutions into individual subfolders (I get emails about this pretty regularly).  For this simple case, there is a correspondingly simple solution.  In your TfsBuild.proj file, just replace:

    <SolutionToBuild Include="$(BuildProjectFolderPath)/../../Directory/Solution1/Solution1.sln">
      <Targets></Targets>
      <Properties></Properties>
    </SolutionToBuild>
    <SolutionToBuild Include="$(BuildProjectFolderPath)/../../Directory/Solution2/Solution2.sln">
      <Targets></Targets>
      <Properties></Properties>
    </SolutionToBuild>

...with:

    <SolutionToBuild Include="$(BuildProjectFolderPath)/../../Directory/Solution1/Solution1.sln">
      <Targets></Targets>
      <Properties>OutDir=$(OutDir)Solution1\</Properties>
    </SolutionToBuild>
    <SolutionToBuild Include="$(BuildProjectFolderPath)/../../Directory/Solution2/Solution2.sln">
      <Targets></Targets>
      <Properties>OutDir=$(OutDir)Solution2\</Properties>
    </SolutionToBuild>

Note that this uses the new ability in Team Build 2008 to pass custom properties into each solution when it is built.  As shown in this earlier post, you can also call custom targets for each solution using the Targets metadata also shown in this example.