Which Mobile Framework Supports Cross Platform Application Development?

When we want to develop applications that should run on different O/S's like Android, iPhone, BlackBerry, Symbian and etc., we need to know what options we have. 


PhoneGap? 
Since winning the Web 2.0 Expo LaunchPad competition in April 2009, PhoneGap has been widely recognized as a game-changer for mobile app development. The open source code has been downloaded more than 200,000 times and hundreds of apps built using PhoneGap are available in mobile app stores and directories. In 2009, PhoneGap was named by InfoWorld as a top emerging enterprise technology in the category "cross-platform mobile app development". The PhoneGap mission is to Web-enable native device functionality with open standards like HTML, CSS and JavaScript so that you can focus on the app you're building, not on authoring complex platform compatibility layers.
PhoneGap Roadmap


RhoMobile?
Rhodes is an open source framework to rapidly build NATIVE apps for all major smartphone operating systems (iPhone, Windows Mobile, RIM, Symbian and Android). These are true native device applications (NOT mobile web apps) which work with synchronized local data and take advantage of device capabilities such as GPS, PIM contacts and camera.
Rhodes


See also:
Whoop - everything mobile they say in just 4 steps! 
Mobinex 


These are just a few options. More to come...hopefully! :-)