From what I have seen on friends unibodys this will happen to those models also (because of what Kyle mentions) but not as much as the older alu models.
I was sceptical to covers but am very pleased with Invisible Shield on my MacBook Pro 15" unibody. I am a fan of the Invisible Shields (www.zagg.com) and apart from keeping the palmrest clean I found the best part is that it is easier to carry the MacBook, it doesn´t slip out of your hand (all the alu stuff does!) and onto the floor. I never slipped i.ex. my iPhone with the shield on it. I would say this kind of shield is ultimate since it doesn´t add any extra bulk to the size.
Only thing I am displeased with is that all Invisible Shields eventually get dirty on the edges and my iPhone looks kinda groce after a year. But peel it off and it looks new. Same with MacBook shield! Invisible Shields are also not easy to apply and fairly expensive. But the day you sell your Mac you will be paid back :-)