When Marco Armant talked about licensing custom fonts for use in apps I did a bit of research myself because I wanted to know the modalities available for us iOS developers.
There are many businesses based on licensing fonts for use on desktop machines as well as web apps. But just to get a general feeling about this I semi-randomly picked MyFonts.com. They also have a fabulous app called What The Font? on the app store that lets you find fonts by snapping a picture of some text.
The font business has not really cared much about apps so far, but now with a Retina display in the size of an iPad this becomes a viable target market for them, as developers are trying to get an unique look by offering great fonts for apps that you read something with.