Responded in your other thread - but here it is again
I'm running FCP5 on my iBook from time to time, since I'm gradually working my way through the lessons in Diana Weynand's FCP5 training book and it's nice to have at-hand for ad-hoc lessons here and there.
It looks like 'mikemodena' has a lot more day to day experience with his iBook setup - ext. hard drive's definately the key.
I'd say if you've already got hold of FCP then go for it, if you're going to invest in Final Cut Studio (as Pro's not available seperately anymore) then I'd seriously consider moving up to a refurb 15inch PBook or maybe a MacBookPro, so you have the optional PC-card slot for an extra FW400/800/SATA channel - allowing you to grow it into a capable mobile editing machine.