Just off the top of my head:
1. I dig the wider screen (for Photoshop and Illustrator palettes) AND the 1280x854 resolution, compared to the 1024x768 on the iBook.
2. Feels a tad more "future-proof" and well-rounded, with the PC slot, FireWire 800, DVI, etc. I feel like I've got a machine that I can use for a good 2-3 years and handle any connection/peripherals that I may encounter. Plus I can hook it to any sort of display, projector, etc. that I may come across the next several years (presentation, buying an external LCD, etc.).
3. 64MB graphics vs. 32MB...matters to some, I'd imagine, depending on what they're doing.
4. Appearances might also count and come into play: sleeker, more "professional" (whatever that means) look. Again, matters to some.
5. Larger RAM capacity (for those who need it, it matters)
6. I imagine some internal, chip-related capabilities/specs are a bit more robust (cache, bus, etc.)? The PowerBook probably has a bit more muscle under the hood for Photoshop or video work. I'm sure it does, in fact.
Don't get me wrong...I probably could've gone with an iBook and been pretty damn happy. But since I'm not murbot and don't tend to buy a new Mac every 6-8 days, I wanted to get something that would handle all the Photoshop/Illustrator/InDesign stuff I throw at it, had the latest I/O and a nice wide, higher resolution display for those palette-heavy apps I tend to spend lots of time in.
Besides, I had a 15" LCD iMac for 18 months, at 1024x768...and I was tired of the cramped clutter on my screen of that particular resolution. I wanted to "break out" a bit and not spend half my time hiding and moving palettes or scrolling around a document.
