Unconfirmed reports of an offline Gmail offering started making their rounds yesterday -- it looks like they began with this Hindustan Times article. If these rumors are true, offline support powered by Google Gears will be launched this year. Google Gears is an open source browser extension that makes it possible for developers to take their web applications offline where it makes sense.
Currently, Google Reader is their only service using Gears, but it seems to be a natural fit for several other services that can currently only be used while connected. An offline component for services like Google Docs, Picasa Web, Blogger and Gmail would definitely make sense.
It will be interesting to see how Google makes this work though -- depending on the number (and size) of messages in your inbox, the amount of space required to store them could be up to 3GB (plus any additional storage you have purchased). Without full-text storage of every message in your account, things like search cannot be included in an offline version of Gmail, and clients like Outlook will remain the preferred choice of business users.
Disk space is cheap, but this problem is likely why Gmail doesn't currently use Gears. The same holds true for services like Google Docs too -- depending on the number of documents, large amounts of local storage may be required to launch an offline version that works as users expect.