Aside from damage which can happen using the wrong cleaner on a particular material, the iPad (and iPhones) have the oleophobic coating - something worth protecting. Apple's official word (btw, it's in the manual...):
Cleaning iPad
To clean iPad, unplug all cables and turn off iPad (press and hold the Sleep/ Wake button, then slide the onscreen slider). Use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don't use window cleaners, household cleaners, aerosol sprays, solvents, alcohol, ammonia, or abrasives to clean iPad. The iPad screen has an oleophobic coating; simply wipe the screen with a soft, lint-free cloth to remove oil left by your hands. The ability of this coating to repel oil will diminish over time with normal usage, and rubbing the screen with an abrasive material will further diminish its effect and may scratch your screen.