you can use HTML to make an app. in fact many of them do, the app just opens an embedded browser and uses code to hide the toolbar at top.
http://www.sencha.com/ is a good place to get started. they even have two of three completely written apps that come as part of their demo. cool shit!
P.S. if you read this and make part of that 19Billion, i want 1%, Please ;)!