I saw an article in Dr. Dobb's this week on the new cross-platform Sencha Touch framework for Apple's iOS and Google's Android based devices. I think anyone doing development for multiple platforms would like the idea of being able to write to one common framework regardless of device, but there is a downside to Sencha's approach, since it can't access the device.
Palm's WebOS, recently acquired by HP, also offers an HTML5 framework known as Mojo. However, unlike Sencha, Mojo is offered natively on the device and it gives the application access to device features like GPS, accelerometers, ambient light, IR, etc. In addition, Mojo allows the user to interact with other native and custom applications and services on the device, like the mapping service (see image).
I am unsure if Sencha's claim to be "The First HTML5 Mobile App Framework" is true or not, but without access to the device hardware it doesn't seem much more than a locally cached web application.