In my testing environment I'm using mongoDB as database system. Therefore the drupal mongoDB module with version 7.x-1.0-rc2 is installed. If I try to run the tests of a module by simpletest framework the progressbar shows up but makes no progress. If I switch back to mysql database, the tests are running fine.
I tested it with drupal version 7.12, 7.15 and 7.16 what made no difference in behaviour.