I recently took the plunge and purchased a 12" iBook, and am officially an iBook enthusiast now.
I needed a laptop for essentially the same things you did, although add video editing to that list as I currently use my iBook as an on-the-go video editing solution so that I can wow clients with the speed at which I can throw their footage together. Thus far, the iBook has met or exceeded all my expectations. While I can't compare these activities directly to a 12" PowerBook, I can at least let you know why I prefer the iBook.
1) $$$ -- for the money, I don't think it can be beat. Being a college student, about to move for grad school, I need a workhorse that wouldn't put me too far in the hole.
2) Aesthetics -- Maybe I'm the only one, but the only PowerBook that I feel looks as good as it should is the 15. The 17" is just too huge, and the 12" looks funny to my eyes when the face is open--something about the way the keyboard sits in there. In general, I also think that the white plastic looks nicer than the aluminum, or whatever the powerbooks shell is made out of. This may not be a concern for you--it was for me, since I do occasionally have to meet with people. The iBook, in my opinion, is a real eyecatcher that draws attention in a conference room full of silver and black laptops.
3) Feel -- Comparing the two side-by-side, I much preferred the trackpad and mouse button on the PowerBook. The iBook's mouse button doesn't have quite as soft a feel. That being said, I prefer the iBook's keyboard--in fact, I don't think I've ever typed on a better keyboard--I prefer it to my desktop's keyboard (an old P2 that I bought 4 years ago).
4) Screen -- While the PowerBook has the better video card and the ability to do higher resolutions, I prefer the iBook's screen. My wife and I drove 3 hrs from Austin to Houston so that we could go to the Apple Store in the Galleria and compare the screens side by side. At maximum brightness, the iBook's screen looks brighter and more vibrant to my eye than the PowerBook.
5) Durability -- I heard great things about the iBook's durability and portability. Someone earlier said the powerbook was more portable. While it is slightly smaller and lighter, I think that the iBook FEELS better in my hands. The iBook is the first laptop that I've picked up with one hand and did not feel like it was going to fall apart or like it was putting any strain on the computer itself. As I understand it, the iBook is made to be lugged around (in my case, inside a backpack all over campus).
I don't know how heavy into PhotoShop you are going. I don't do too much that requires long render times....but I do quite a bit of video, and have even used After Effects on the iBook. It is definitely not a snappy process, but it's not even snappy on the G5 PowerMac at work (although, much better than the iBook, obviously).
In the end, you get what you pay for. The PowerBook is definitely a better computer in terms of actual computing power. However, you need to way whether or not the extra cost is worth it--in my case, it definitely was not.
Someday, when I land that cush job, I might switch over to a PowerBook (especially if they have G7s or something crazy like that in them). But for now, while I'm tromping around campus and jostling my computer about inside a backpack--I'll take the added durability of the iBook.
Just my two cents...remember that in most cases I do not have a direct head-to-head comparison with the iBook. Also, it should be clear that I have an iBook bias!!!
Either way--it's an apple and it will work...yea!