Differences:
New powerbook has all the following that the ibook does not have
1. Faster hard drive (powerbook = 5400rpm, ibook = 4500rpm)
2. Faster bus speeds on powerbook
3. More RAM (if your going for standard configurations) 512Mb on one slot rather than 2, so upgrade potential is better.
4. Slightly faster CPU
5. Digital video output, for connection to say a tv or projector
6. Audio input
7. Higher resolution on 15" powerbook, so you can get more on the screen
8. Double the graphics power
9. Gigabit ethernet for faster network connections etc.
I had the dilema of picking between the two, and I settled for the powerbook model discontinued in January of this year, the powerbooks specs of my one are slightly better compared to the new ibook. But the extra screen resolution, built in bt, larger hard disk and better build quality swayed me. You dont get digital video out, better graphics, audio input on ibooks and if your into multimedia like me they will come in use. Also the slightly faster bus speed will make a difference in performance.