We've long been recommending Google Apps for your domain as a great email and calendaring solution. Further integration with Google's APIs has been "on the list", and now that Google has announced their Premier Edition, it looks like they're rolling out official ways to extend and integrate with the Google platform.
The Premier Edition is pricey (not expensive: it's in line with costs for big, Enterprise scale applications and support) at $50US/user account/year. How can open source work with this system? Can only the Premier Edition be extended with partner solutions? We'll be following this quite closely and looking at how our favourite open source project, Drupal, can work with these new Google features.
I've written a bit more about this topic over at my personal blog.