This is what Apple says.
Handle your iPad with care to maintain its appearance. If you are concerned about scratching or abrasion, you can use one of the many cases sold separately. To clean iPad, unplug all cables and turn off iPad (press and hold the Sleep/Wake button, and 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. iPad has an oleophobic coating on the screen; simply wipe iPad’s 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.