I've heard that Google is tightening up on Droid apps. iPhone and Windows Phone 7 are tightly controlled platforms. So while some independent sales are still possible, the great majority of developems would have no possibility of making a profit that way. They couldn't price their apps high enough, given the small number of potential buyers they could reach.
Because of the risk of malware, most people wouldn't buy phone apps from unknown sources.
So the situation is really very comparable I think. In both cases the issues are sellers needing ways to connect with buyers, and buyers needing ways to verify the quality of products before purchase.
Rejections are also a big issue. A developer can spend months on an iPhone app only to have Apple say "mmmm.... no. We just don't like it."