I demo media apps on my site, but to be honest, there is no way I can demo all the paid apps -- like paying $17.99 a month for Wall Street Journal access. (I assume their app will be free, but worthless unless you pay the subscription price through the app.)
One way to see these apps before buying is to see if a third party vendor created the app. For instance, Handmark created the Daily News app and others. Of course, most of these apps are free, so I'll admit that demoing fee apps is pretty much a waste of time.
Another solution would be for both the Apple stores and Best Buy to load up their demo iPads with as many apps as is practical. I wrote on Friday that one thing I'd like to see is more app developers (I was talking about media companies) working with Apple and Best Buy to market their apps instead of just having iTunes do the work (which means they simply get lost in the app store).