Wow, I was worried about my sale volume estimate being overly optimistic, but I have the most pessimistic estimate here. I figured one in ten iPhone/iPod touch owners would see any given app in the store (a number of people won't ever use the store, and most won't check the store on a regular basis), and then out of that one in ten, one percent actually go on to buy the app (which is typical in the shareware industry (yes, I know this isn't the shareware industry, but the number seemed fair)), which results in selling the app to 0.1% of the iPhone/iPod touch market.
Going off of that estimate, a typical app would probably sell to 16,000 people given the 16,000,000 device estimate. At $4.99 a copy (which is what I'm planning to sell a game at), that results in $55,888 after Apple takes its cut. If you release a game or an app once every six months that sells that much, you can live comfortably with iPhone development as your only source of income.
So, yeah, even with my pessimistic estimate, things look pretty good for iPhone developers ^_^.