I sadly have no direct experience regarding powerbooks, but from general experience with high performance laptops, running windows, you’d have to be paid either really well, or be “on the road” for literally weeks for it to be worth spending that much on a laptop computer, instead on a desktop solution.
I have no doubt that performance will be up to par, given the specs, but I’m just urging you to consider the efficiency of buying what in the end, is a system that trades performance for portability, and costs more in doing so.
Some of my colleagues however at the university have been using the new powerbook for general webdesign and graphic design purposes with Adobe CS5 , and it’s perfectly functional, even with heavyweight files.
Hope this helps, and good luck with your purchase! 