My wife and I are graphic designers and we just purchased a high-end 15" (should be shipping sometime today...
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I would strongly recommend avoiding the iBook unless you're planning on doing some extremely light graphic design; I don't know about you, but when I do web development, I've usually got Flash, Photoshop, Thunderbird, Textpad, and two or three Firefox windows open. There's no way an iBook could handle this gracefully; Photoshop CS alone would be a tall order.
When looking for a Powerbook for graphic design, we discussed two options:
- a 12" model with plenty of RAM, a big external monitor, and a wireless keyboard/mouse (so when we get home we can just plug the pb into the wall/monitor and have a decent desktop machine)
- a high-end 15" model
The reason we broke it down into these two choices is mostly because of price. We definitely couldn't afford to spend more than $2000, and both of these options turned out to cost roughly that.
If portability is a really huge issue, I would go for the 12", but you're going to regret it if you do any serious graphics work. The 12" just isn't suitable for heavy duty graphic design, purely from a real-estate standpoint.