I think this is the major thing that Google is lacking, an true application development platform that anyone can build an application, host it inside of Google, and use internal data sources.

App engine feels like it's close to that, but it doesn't confer any special properties onto applications developed on that platform. Why can't I register a widget inside of Gmail or Google Calendar? Why can't I integrate a workflow form or something else into apps without developing a standalone application that only integrates through an API?

I'm hoping this is where they're going with Dart, creating a language that can be compiled into Javascript and share the backend connection with official Google apps. SPDY and WebSocket will enable these types of applications, as they won't need independent connections for each API request channel.

I can see how Google is cautious about this type of solution, given that it opens a huge XSS footprint, but I think it's the future of the "cloud" applications. For these platforms to become substitutes to native platforms they need to support applications that integrate as deeply as the first-party offerings.