I've updated The ThreadTester Library with a new ability: StopWhenTrue().

It allows you to periodically poll while all the threads are running (and the test is blocking) and either assert stuff or stop the run midway when some condition is true. 

• Download source and binary.
• Here's the source in SVN.

Here's a test that shows how this is possible with the new API:

[Test]
public void StopWhenTrue_ReturnTrueWhenCountOver1000_AllThreadsStopImmediately()
{
 Counter c = new Counter();
 ThreadTester tt = new ThreadTester();
 tt.RunBehavior=ThreadRunBehavior.RunForSpecificTime;
 tt.AddThreadAction(delegate
             {
                for (int j = 0; j < 103; j++)
                  {
                                               c.Increment();
                                           }
                                           Thread.Sleep(50);
                                       });
 
 tt.StopWhenTrue(delegate
                  {
                    // you can also assert stuff here
                    Console.WriteLine("currently at " + c.Count);
                     return c.Count > 1000;
                  },100);// check every 100ms
 
          //this test will end in less than a second because of our added functionality
tt.StartAllThreads(10000); 
 Assert.Greater(c.Count,1000);
 Assert.Less(c.Count,1050);
}