No, totally untrue. The installer for Final Cut Studio 3 is NOT PowerPC, it is native Intel. It won't even install on PowerPC Macs! The "installer" is in no way shape or form PowerPC based. Who ever told you that is a moron. If it were true, how did I install it in Snow Leopard with no Rosetta installed?
I do a ton of consulting and training, and can assure you that every Apple retail store person who talks about Final Cut has no clue what they'er talking about. I've had lots of Apple retailt store former employees come into my classes, say they were FCP "trainers" in the store (so why are you in my class?), and half way through the first day are totally blown away by what they don't know, and are misinformed about.
"Genius at the Apple Store" only applies to iLife, not pro apps. They are not pros, leave pro apps to us pros.
Buy a new Mac with Lion on it, install Final Cut Studio 3, then install FCP X, no problems. Just be sure to launch Motion 4 BEFORE you launch any other Final Cut app once the installs are done. Then you'll be fine.
"The installer is PowerPC based." What brain dead idiot told you that? Totally false. In fact, I know someone running Lion (dev beta) on a MBP and has FCS3 and FCPX running side by side just fine. The install was just fine.
FCP 7 will run perfectly fine in Lion, Apple has verified that publicly, period.