Our dear friend Erica Sadun has outlined one of Apple's more sticky App Store policies over at Ars Technica. There's been a lot of customer pressure, as we've said before, to put "try it" versions of apps on the App Store, and quite a few developers have done exactly that, by releasing a "Free" or "Lite" version of the paid app along with the real thing. But Apple has some pretty strict rules about doing so: every app on the store needs to be fully functional and stand on its own. You can leave out some levels of your game, for example, but you can't put a timed limit on it or remove features that are central to the app itself.
Of course, there's a grey area there -- what features are central to the app, exactly? But Erica boils it down to words: putting "Lite" or "Free" in the title are fine, but releasing "Demo" or "Beta" versions are not.
It's too bad Apple hasn't fleshed this out more -- everything we've heard about how people are buying these apps shows that if customers have a chance to try the app, they're much more likely to spend money on it later on. But right now, developers have to keep up two different versions, and they have to dodge the pitfalls of deciding what goes in each one, while keeping them both "fully functional."