I'm pretty sure there is, as of yet, no way to run a second copy of some OTHER MacOS X while booted in X.

You can run MacOS 8.6 or 9 (at least 9.0.x) using the emulator SheepShaver, which has indeed been ported to the Mac and runs as full-speed (uses the native Mac processor instead of having it emulate an older PowerPC processor).

You can, of course, boot MacOS 9.2.2 in Classic, which will let you run most MacOS 9 software, but you'll be in the MacOS X Finder environment.

Mostly, though, make MacOS X your own. It's a good OS.

Just a question about SheepShaver... on the official page it states that it is available for Darwin... not OS X... Is it just that the version for Darwin will run native in OS X with no mods ? (aking as I've had bad surprises about this in the past...)

No, there's an Aqua port. Doesn't mean you can't run the Darwin port, but the Aqua port doesn't require X11 at all.

Cow Pie
 

Hmm. The future naming has potential... MOOSX. :D