I just tried FCP HD with Tiger and it seems to work just fine, though I didn't do a whole lot of testing on it. I dropped a clip on the timeline, scrubbed around a bit, added in a cross-dissolve, and that all worked perfectly.
However, I wouldn't recommend installing Tiger with FCP HD on a mission critical system where you'll have a deadline too soon - as there could always be unforseen difficulties with it or with other programs (QuickTime, Compressor, DVDSP, Motion, etc.)
If you don't have a deadline to meet and are the adventurous type, fire away! 🙂