My machine has a multi core processor, but my unittests only run one at a time.

In Visual Studio 2010 the option to use more than one core and run unittests in parallel is there, but well hidden in the testsettings. You can change the default behavior (no parallel execution = 1) to automatic (0) or the number of cores you want to use. Open the local.testsettings file from the solution explorer with the XML editor. Locate the Execution tag and add the attribute parallelTestCount=0 to enable automatic parallel execution when available.

After setting the parallelTestCount to automatic my unittests run in parallel on my dual-core machine. You might want to restart Visual Studio or do a clean build.

The execution time of the 17 unittests in my solution was sliced in half this way: 2 seconds to 1 second.

Read some of the details what is and isn’t possible on the MSDN blog post about executing unit tests in parallel: http://blogs.msdn.com/b/vstsqualitytools/archive/2009/12/01/executing-unit-tests-in-parallel-on-a-multi-cpu-core-machine.aspx