According to the Hindustan Times, Google plans to release an offline version of Gmail. I think that it's safe to assume that it would run on Google Gears, much like Google Reader's offline feature does.
Obviously, the lack of offline e-mail reading is the one huge downside to using Web mail. If Google implements this correctly, it could be a pretty big deal, but we will see if the functionality of a browser-based offline solution can compare to an actual desktop client.
It should be noted that Windows Live also currently has its Live Mail Desktop client available right now (one of my favorites, by the way). Whether Google goes browser- or client-based with its offline functionality, this is what it has to compete with.