Trialware could be tricky to integrate into the current management, but surely free demos would be simple to do? Like the free apps we see already on ITMS? Where you've got so many titles with "xyz LITE" and "xyz full/pro".
And a lot of the time the only difference between the LITE and full versions is one is ad supported and the other is bought and has no ads.
From a developer standpoint, releasing and maintaining two codebases is something of a pain in the ass, even if they share a lot in common. And you have to go through the submitting and review process twice to get 'em both in the app store.
In the past, nearly all trialware and shareware, from my own little programs all the way up to Adobe Photoshop, has had a single program that you download and install, and if you like it you buy a serial number that unlocks it or extends the trial period or whatever whatever.
Unfortunately, that model doesn't really work with the App Store system very well; the App Store system requires a payment up front, before download, so downloading a free app which you then go to the developer's web site to pay for and unlock actually violates the App Store Ts & Cs (for obvious reasons).
What *could* be done, though I haven't seen it personally yet, is you could use Apple's in-app-purchase API; you'd download a free version of the software, then to register/unlock it you'd make your payment via the app's own in-app purchase system. That'd require a bit more coding, but probably less than maintaining two separate apps.
