Hi John,
NCrunch doesn't yet have the latest version of NUnit (2.6.2) packaged with it, so it won't work with NUnit async tests straight out of the box.
However, something you may want to try is updating the NUnit binaries used by NCrunch. Replace the following files with their equivalents from the latest version of NUnit:
\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\Extensions\Remco Software\NCrunch for Visual Studio 2012\nunit.core.interfaces.dll
\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\Extensions\Remco Software\NCrunch for Visual Studio 2012\nunit.core.dll
A quick test from my side shows that this allows the async tasks to work correctly. I haven't yet had a chance to exhaustively regression test compatibility with the new version of NUnit, but the scope of changes in this revision doesn't look big, so hopefully replacing these files won't surface any other problems. I've made a new to include the newest binaries in the next minor revision of NCrunch.
Cheers,
Remco