With the release of Froyo, Google promised to start decoupling applications bundled with the Android at the core - we've already seen it done to Maps, Navigation, Street View, and other programs. Today Gmail joins the party with the rollout of a new version 2.3 that can now be installed from the Market.
What does it mean? Well, previously, the Gmail app was only updated when you upgrade the whole operating system. Now that the app is downloadable from the Market directly, any updates will be available immediately.
Unfortunately, only users of Froyo and above are able to install the standalone version - everyone else is out of luck.