At one point, when I had too much to support, I had 2000, 2005, 2008, 2008R2 and a beta of 2012 all running on the same machine (with two different versions of Express too). You can't expect performance or good behavior with this type of setup, but it will function.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning