Mat, Aaron's out of the office right now, but I'll make sure he sees this when he returns. :-)
Unfortunately, the App Store specifically disallows the sort of time-limited demos we do on the Mac. We feel that would be the best way to demo OmniFocus, but it's not currently possible. Instead, we try to make sure folks have
plenty of information available to them before they purchase, and we back their purchase up with a
30 day money-back guarantee.
We're are working on OmniPlan for Mac 2.0; I don't know exactly what the beta plans are there - we've generally done public betas in the past, but I don't want to make specific promises since I'm not actually on that team. :-)
Similarly, we are going to do OmniPlan for iPad, but we don't know exactly when it'll be ready. When we do know, we'll definitely make an announcement.
I think the best thing to do in the short term is to send an email (if you haven't already) to
omniplan@omnigroup.com and make sure they know you're interested. Thanks!