The Tech Erra points to a snippet of code discovered in iTunes 10.5 beta 9 indicating that Apple may begin allowing App Store app rentals.
"510.036" = "Apps are automatically removed from your iTunes library at the end of the rental period.";
This either suggests iOS users could finally have the ability to play around with an app before deciding to purchase, or someone just got sloppy with some strings.
Personally, I've desired some type of demo capability in my iPhone ever since Apple debuted the App Store in 2008. A "try before you buy" approach could mean more sales for developers and a more pleasurable experience for users. But rental? How much would it cost to "rent" an app, and for how long? 15 cents for 24 hours as opposed to a full 99 cent purchase? Seems a bit awkward..