Sorry, FCP 5.1 won't work with Mountain Lion either. The installer...INSTALLER...requires Rosetta. Only FCP 7 will install on Mountain Lion. So...looks like eBAY is in your future. or another NLE option, like Adobe Premiere.
Hey Shane. I do have a Snow Leopard machine as well (not to mention the original G5 that FCP 5.0.4 is on currently), so presumably the installer would work on either of those Rosetta equipped options [edit: Rosetta not needed on the G5 obviously].
Bummer about 5.1 not being able to run in ML though.
eBay = $1,200, give or take. I was hoping for a substantially less costly solution...