The demo application included with the SDK shows exactly what can be done with these new technologies, and it looks impressive.
The Opera Mobile Platform in action...
Of course, being me, the first thing I tried to figure out how to develop under Mac OS X. It looks like the only tool from the Symbian SDK you need is the
makesisapplication, which compiles just fine under Mac OS X,
tar -xvzf makesis-2.0.0.tar.gz
...and since Opera provide a copy of their Platform-enabled browser for Mac OS X, so you can test your applications on your desktop machine before using bluetooth to deploy them onto your mobile device, it looks like cross-platform development under Mac OS X is pretty well supported.
Update: Opera are hosting a forum for discussion of their mobile platform. Not much content there yet, but it does look like a couple of people from the development team are reading it, so it could turn into a good resource.
Update: A lightning update for the Application Framework...
Update: More from Ajaxian.com on AJAX on a Pocket PC...