ReadWriteWeb has a post up about the fact that Zoho has added offline access to Zoho Mail using Gears. Zoho continues to do a really good job of innovating when it comes to Ajax-based RIAs. They were very early to the game in creating offline support for Zoho Docs before Google beating them by almost 5 months.
As I've mentioned before, the hardest part of the online/offline problem is synchronization. I'm not exactly sure how Zoho is implementing that part of offline Zoho Mail but I have to wonder if one of the reasons that Google hasn't added support for Gears yet is that they're spending a lot of time on the synchronization problem. Both Adobe and Microsoft have products that help handle online/offline synchronization. Adobe has LiveCycle Data Services and Microsoft has the Microsoft Sync Framework. It wouldn't surprise me at all if Google was working on something similar and looking at how to bake it into Gears.
An open source synchronization platform would be a big deal and would go a long way towards blending the browser and the desktop. In a lot of ways I think that's one of the major problems that has to be solved before we move on to the next generation of RIAs. I've been doing some thinking about the cloud and RIAs recently but you can't fully jump into that unless you make it easy for developers to use the same piece of data wherever they are.