Glass is pretty scratch-resistant. You can take your keys to an iPhone or iPad as vigorously as you'd like with no ill effects. I've demonstrated it to several people, to usually shocked responses. Cat claws don't do anything to windows, they're not going to hurt an iPad.

This is also part of the reason why Apple's stopped selling screen protectors - they're as useless as Monster cables. They don't actually help anything, and they get scratched easier than the screen does.

As long as the cat's nails have a lower Mohs hardness [1], they won't scratch the screen. In this case, glass has a mohs hardness of about 5.5, and karatin (what nails are made out of) is about 2.5.

[1] http://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness

Metals can't scratch glass. Cat claws have even less chance.

"Q. Do metal scrapers or razor blades scratch glass. A. No. Metal scrapers and razor blades alone will not scratch glass. However, abrasive materials, such as sand or fabricating debris from the tempering process, trapped by a metal scraper or razor blade, can scratch glass."

The glass on iPhones, iPod touches and iPads is quite scratch resistant (it is, after all, real glass). Scratches are possible but unlikely. Small sharp stones (sand!) are probably the likeliest thing to scratch your iPad and I think cat claws don’t have quite the same properties.