Erik Starck writes that it will be the Internet.
The majority of mobile applications will be web, web, web and then maybe some Java, XHTML or Flash Lite on top. Developers used to developing for the PC web browser might think of them as thin clients. Mobile phones on the other hand are ultrathin clients. This means the server has to do more work and there has to be two different GUIs to the same service: one thin and one ultrathin.
The problem with todays phones is not about access to the native OS, but rather how the web or downloaded applications are second grade citizens within the phone GUI. This, however, will change.