I've edited a series of projects with FCP 3 (although none of them longer than 25 minutes) on my iBook 500 with an 40 gig FireWire drive, and I must say that aside from rendering times (which I don't come across often, anyway), the iBook's fine.
Recently I've been getting a little bit of REALLY bothersome stuttering when outputting back to my camcorder, but I'm pretty sure it's just because my drives are rather in need of defragmentation.....now if only I could install Drive 10 on my FireWire HD and manage to have it repair the differently administrated setup on the iBook's internal drive....
But that's off-track!
In summary....yes, the iBooks are okay with FCP 3, as long as you stay away from too much title/effect rendering.
And in terms of GUI responsiveness and perceived speed, FCP is RATHER faster than iMovie 3, which I dread running on my machine, just for the record.