From their site:
Creating advanced applications using open standards Web technologies is significantly easier and more-cost-efficient than using traditional technologies. Because Opera Platformâ„¢ is based on Web technologies, any Web developer can quickly and affordably create new and advanced mobile applications with a remarkably short time-to-market. Existing Web applications can easily be adapted to fit the mobile phone. ...
The cool thing is they've synced their releases, so you can download their regular browser to do your widget development on your desktop (even on my Mac!). This is pretty damn neat if you ask me. I'm definitely going to mess with this thing and see what I can come up with.