You need a (fairly expensive) "ADC" adapter for Apple displays. That's because they use ONE cable for power and data (including the display's USB hub). Your PowerBook lacks the ADC connector the PowerMacs have, so adding that means connecting a box to your video-out and USB ports, as well as to wall power--and then the one cable to the display. ADC is very convenient--for a desktop. Less so for a laptop that needs an adapter. I hope Apple's next displays come with DVI support as well as ADC.
So a 3rd-party DVI display would tend to be a cheaper option.
But note that when shopping on Apple.com, if you config a PowerBook and add an Apple display, the price listed ALREADY includes that ADC adapter. (Try adding the same display to a PowerBook order, and you'll see it's less.)