This dramatically changes the economics of the App Store. Until now, productivity apps could charge up front as paid downloads and that was it. Updates had to be free, or, to charge for major new versions, developers would have to play confusing games by making the new version an entirely new app. Twitter clients like Tweetbot and Twitterrific, for example, did this, to justify years of ongoing development. Now, apps like this can instead charge an annual/monthly/etc. subscription fee.
This could be the change that makes the market for professional-caliber iPad apps possible. On the Mac, there has long been a tradition of paying a large amount of money for a pro app, then paying a smaller amount of money for major updates. The App Store has never allowed for that sort of upgrade pricing — but upgrade pricing is what enabled ongoing continuous development of pro software. Paying for each major new version, however, is arguably a relic of the age when software came in physical boxes. Subscription-based pricing — “software as a service (SaaS)” — the modern equivalent. That’s the route both Microsoft and Adobe have taken.
I don't see this affecting most mainstream consumers very much; most people really only use five apps, and those apps are usually the free, popular and cross-platform ones.
But what I am excited about is how this helps power users and productivity apps. Currently, most people would never take a chance on a $20 app. But with the new subscription-based pricing and free trials, that barrier is greatly lowered. Less commitment for consumers and more sustainability for developers.
I'm optimistic about real desktop-class apps coming to iPads and iPhone.