This functionality, which will allow people to use the Gmail interface when disconnected from the Internet, has been expected since mid-2007.

That's when Google introduced Gears, a browser plug-in designed to provide offline access to Web-hosted applications like Gmail.

Gears is currently used for offline access to several Web applications from Google, like the Reader RSS manager and the Docs word processor, and from other providers like Zoho, which uses it for offline access to its e-mail and word processing browser-based applications.

Rajen Sheth, senior product manager for Google Apps, said that applying Gears to Gmail has been a very complex task, primarily because of the high volume of messages accounts can store. "Gmail was a tough hurdle," he said.

Google ruled out the option of letting users replicate their entire Gmail inboxes to their PCs, which in many cases would translate into gigabytes of data flowing to people's hard drives. It instead developed algorithms that will automatically determine which messages should be downloaded to PCs, taking into consideration a variety of factors that reflect their level of importance to the user, he said. At this point, end-users will not be able to tweak these settings manually.