I almost bought a 12" iBook two years ago, but my dad convinced me to save a little more for an iMac. When I look back on it, I can't possibly imagine using FCE on a 12" 4:3 display. Sure, a 12" screen may be fine for people like my sister, (PowerBook) but I can't imagine using FCE on anything other than a widescreen. I think the new 13" widescreen MacBook is the sweet spot in size. 15.4" is a great size, and I prefer the aluminum on the MBP over the polycarbonate plastic on the MacBook, but 15.4" can seem a tad too big.
If they had a 13" widescreen MBP, I'd probably buy it, but the 15" is not only a tad too big but a tad too expensive as well. Plus, the MacBook and MacBookPro are closer in terms of features than the iBook and PowerBook two years ago when my sister, my brother, and I were buying new macs. Portability wasn't a major thing for me two years ago, so I bought an iMac. But my brother and sister need portability, especially my sister, and the iBook's crappy 1.2 GHz and 1.33 GHz with 256 MB RAM couldn't stack up to the PowerBook's 1.5 GHz and 1.67 GHz with 512 MB RAM (and much bigger hard disks.) Had the MacBook and MacBookPro been out when they were looking, we probably would've gotten them a MacBook, but their PowerBooks have suited them well.