One gotcha that got us was testing 2005 databases.

The test we wanted to do was see how a large database performed on the new 2008 machine. We detached it from the 2005 server, attached it to the 2008 server, did some tests and then detached it to move it back to the 2005 server. The 2005 server would NOT let us attach the DB saying that the file version was above what it expected -- ie the files had been changed to the 2008 version, even though we were running in 2005 compatibility mode. It's a one way street...

Had to restore from backups, which were luckily rather fresh.