I had the 12 inch iBook for a year, and for the purposes of writing 5 + hours a day, the small screen size wore me down, drove me crazy, degraded my eyes (plus, I usually had more than a single document open at one time). I definately don't reccomend the iBook for writing any significant amount. That said, I imagine the 12 inch PB is sexier, quicker, and with expose of Panther, more useful for a writer.
But for me, the 17 inch was the only way to go. with my slender Booq carrying bag, it is highly mobile, and I have yet to find a place I can't use it where I used to use my 12 inch iBook. Really, I can't tell you how much I love this machine.