Leslie at jinsync.com released an apollo gmail app that showcased how easily existing AJAX application can be wrapped and deployed onto the desktop. The concept is very simple, use the HTML capabilities within apollo sdk to wrap around an existing web site. Many will say, "so what? I can just bookmark gmail using my browser, and place a link on my desktop as well." Now, look at apollo's documentation. Offline capabilities support within the SDK. Add caching and offline capabilities onto the existing AJAX-based gmail, and you will have a powerful desktop emailing tool without significant rewrite of the entire code base.
Google, pay attention. With Apollo, gmail can be a desktop email client that is cross platform, based on the same code base as the existing web version, and ready to be deployed within a couple of weeks.