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.
The only thing worse than having a proprietary API to interact with your application is to have a proprietary API that can only be run from a proprietary sandbox. On the other hand, Google could make APIs publicly available so someone running on whatever platform could easily call them. This doesn't solve the security problems, but if Google can't trust external people with the API a sandbox isn't going to do a lot of good.