Use One User Account for Snow Leopard and Lion (Partitioned)
Unfortunately, at the moment Lion is still in pre-release phase, so I would not deem it to be safe nor trustworthy (until its officially released).   Even Apple's own guidelines for installing pre-release operating systems to testers/developers say to install it on a non-critical hard drive in case there are problems.

If you want to test it out, I would install it on a completely separate hard drive.  And without any critical data.