Google Wave App StoreIf Google Wave takes off, developers will many build useful additions and extensions using the API. I can imagine that some of these will be so useful that users would pay for them. A third party could then develop an App Store that sells themes, extensions, and subscriptions to extensions. They could then take a 30% cut, much like Apple does for the iPhone.
Verdict: Technically, I imagine that this would be pretty easy to implement for gadgets, and hard for things like themes or elaborate applications - I'd have to see the source code to really judge. The risk here is that Google could do this first. I'd expect they could do this better than any third party could.
Random thought: It would be interesting to know how the Cydia Store is doing, which is a less restrictive, third party App Store for the iPhone.