Apple has complete control over the App Store and its component parts. I’ve bemoaned this control in the past, but it could easily be used to benefit developers and customers alike. Hopefully, as the App Store continues to mature, Apple will consider making it possible for developers to offer free trial versions of their software.
A number of developers are, in fact, already doing this.
They’re doing it by issuing both a “light” or “limited” version of the App for free, and then encouraging you to upgrade to the full, paid version.
(and am wasting a number of evenings with it right now…. highly recommended, if you don’t want a life)
So you can do this today, at least with some classes of Apps, and I’m not sure just how much a formal “trial version” set up might help the cause here; I don’t think it’ll make developer’s lives easier, in any event.
It’s not as convenience at the Xbox 360 marketplace download&activate model, but it’s actually workable and practical today.