Just had a chance to play with an 12" i book this weekend. They are quite nice for what you pay for them. Not the fastest things on the market, but certainly will get all you want done... and do it in style. The new panther OSX revision is supposed to be faster than jag (though I was using jag). Itunes is great as are a lot of mac programs. Panther also now has an X window program that you can run any linux software in... which is nice quark I guess (you can use open office). Anyway, as far as the ibook vs the powerbook... well both come with panther, both use g4 processors (800mhz for the i and 1ghz for the pb), the ibook has 256k l2 and the pb has 512k l2. The new powerbooks come with 256mb ram integrated to the mobo and the ibooks have 128. Both can handle 1 more gig in their 1 bank for additional ram (though apple says the ibook can only use a 512mb stick for a total of 640mb this has been proven wrong). Both use the same rez screans to my knowledge and the ibook actually has a better gpu (mob rad 9200 on the ibook vs FX Go5200 on the powerbook both 32mb) if memory serves me right. The powerbook has the option of the superdrive if you need a dvd burner on the fly, but both are standard with combo cdrw drives, the pb also has a larger standard hdd by 10gb. Finally... both have usb2.0/firewire (400)/and are airport (wireless g) ready... the pb will come with bluetooth standard.
Personally I am considering both, and I can tell you that the powerbook should be plenty fast for your needs (with a sub $100 512stick of additional ram just incase). The machine i'm using right now is a 1ghz g4 w/ 256k l2 and 1gb of ram... and it's plenty fast for my needs. Though it isn't as fast as my personal windows based machine (yes I don't own the mac) it would make a very competative laptop (and I suspect the powerbook would be just as fast if not faster). BTW... they are truly beautiful machines... and the build quality is great... even if some tell you the ibook is too "plasticy"... it's definitely not... it's solid...and lite... and compact.
BTW... i'm sorry for any grammar/spelling erros....that's a long post and it's late...