
Opera Software has released a beta version of Opera Platform SDK, a software development kit for developing and running Web applications on mobile phones. The Platform supports XMLHttpRequest, a basis for creating AJAX applications.
More details here
Excerpt:

Opera Platform architecture
Opera Platform architecture consists of three parts:
- At the lowest level is the Application Player, an extended version of the Opera browser. It provides web applications with access to native phone functionality such as messaging, calendar, battery and signal status.
- Above the Application Player is the Application Framework, which supports interaction between installed web applications. It also offers predefined UI elements, such as menu systems and dialog boxes to ease application development, according to Opera.
- On top of the Application Framework, are the web applications themselves. These are created in established, open standard technologies such as HTML, CSS and JavaScript, Opera says. They have access to the phone's functionality through the Opera Platform DOM interface, provided by the Application Player, and communicate with servers using XMLHttpRequest.
A beta version of the Opera Platform SDK supporting Windows Mobile 2003 Second Edition and Smartphone Edition is available for free download here.