Generally, I avoid case/protectors etc like the plague on phones, ipods, everything - personally, I think the use of them seriously degrades the experience. I've seen people with invisible shields on the *base* of their laptops, which utterly mystifies me. Yes, you could argue higher resale price, but I'm sure I've sold used, un-shielded lappy's for the same price 🙂
The iPad is the one exception, not really for protection but I just find it a lot more practical in the Apple case. If the iPad was somehow clamshell in design, ala the aborted MS Courier, I'd ditch the case.