Many of these apps were ported to the iPhone on the basis of previous success on dumbphones. I don't know what it's like in the US, but these were a big business in Europe. Jamba/Jamster was a big name and their adverts on music television have been bugging me for years:

http://en.wikipedia.org/wiki/Jamster

I know that the Swiss Code Monkeys (http://swisscodemonkeys.appspot.com) have been doing this for about 2 years now. They've also done http://AppBrain.com, so they're also doing more ambitious projects too.