There are no demos available in the App Store, we know this. No try before you buy, no download now, pay later. According to Erica Sadun at Ars Technica, even mentioning demo in your verbiage, like beta, will get you a swift kick in the rejection button. Free "Lite" versions are the only way to go.
Zach Epstein over on the Boy Genius Report wonders out loud why Apple, in its infinite usability, can't create a mechanism where Apps get downloaded free, and then expire after a short period of time unless you decide to buy and keep them.