Sencha Touch is a pretty new mobile web app framework that apparently supports Android, iPhone, iPad, etc. Sencha is the framework formerly known as ExtJS.
Here's a demo of the framework: http://www.sencha.com/deploy/touch/examples/kitchensink/
Note: After looking at some screenshots, I'm not sure that the framework results in apps that follow Android UI patterns very well.
Update: Also check out the just-announced jQuery Mobile project.
