In order to cut down build time and stick it within the famous 10 min limit, testslicer has just been released.
The idea behing the scenes is to detect which classes affect which tests so changes in such classes would trigger only those affected tests.
This was achieved in Java by using javassist to manipulate the user bytecode and ant tasks to allow easy test picking.
Unit test builds time were cut down usually in 70% (depending on your changes, of course) and end-to-end tests is supported though not yet fully documented.
This can be a new way to cut down build time without requiring the installation of grids (as selenium grid still has some issues) or paid software (as cruise)