Children and pets are known PowerBook killers. It's not just the hair to be concerned about. An iSkin does elliminate any foreign matter getting under your keyboard and into your computer. Anything from dust to drinks. It also elliminates a lot of the wear on your keyboard keeping your PowerBook in pristeen condition. I would go for the iSkin and peace of mind.
On the same note, opening the PowerBook and cleaning under the keyboard shouldn't void your warranty as long as you are careful not to leave any tell tale signs of having been opened.