It depends a lot on the specific platform. In fact, this is one of the fantastic things that we've seen developing.
We've seen a proliferation of these platforms, such as the App World for RIM, the App Store for Apple, and the Android marketplace for Android-based devices, all of which are subject to their own philosophies and each of which has benefits and drawbacks.
What happens, for example, if you submit an app to the App Store is that it goes through a pretty rigorous quality assurance process, so that when it finally makes it into the App Store and you can purchase it, you can basically be assured that it's going to work, and so forth. The drawbacks of that is it means that Apple acts as the gatekeeper. It has a series of policies that it can implement and there are circumstances where it won't let apps through, which has led to some complaints.
The Android, on the other hand, is a much more open marketplace and, as a consequence, it's a bit more like the wild west. Essentially the apps that make it into the Android marketplace don't go through this kind of vetting process, so as a consumer buying the app, you're frankly taking a bigger chance, because you don't know if it is going to work or not. However, my understanding is that Google has policies in place that you can get a refund or a return, if things aren't properly functioning.