If you live in Gmail, but don’t always have a broadband connection available, today should be a happy day for you. Google is rolling out a new system for letting Gmail users access their accounts offline. Google will cache your messages on your system using Google Gears. You’ll be able to open your browser to Gmail.com, see your inbox, read and label messages and even write replies without a Net connection. Your messages will send once your system reconnects to the Web.

The system is beta (of course) and accessible through Gmail Labs. But it won’t be immediately available to everyone – Google is parsing out access as it experiments with the new feature. I don’t have access to the new feature yet, so I’ve still got lots of questions. But Google’s post makes it sound like the experience will be almost indistinguishable from using Gmail normally.

“Gmail uses Gears to download a local cache of your mail. As long as you’re connected to the network, that cache is synchronized with Gmail’s servers. When you lose your connection, Gmail automatically switches to offline mode, and uses the data stored on your computer’s hard drive instead of the information sent across the network. You can read messages, star and label them, and do all of the things you’re used to doing while reading your webmail online. Any messages you send while offline will be placed in your outbox and automatically sent the next time Gmail detects a connection,”.

There will also be a “flaky connection mode” that’s supposed to give you the best of both worlds. It’ll assume that you’re disconnected and use the local cache to store your data, but whenever your connection is working, it’ll sync with Google’s servers in the background.

When Google launched offline GMail they promised it would act almost exactly like regular Gmail. From my early testing, it seems like that claim isn’t entirely true — in some ways, offline Gmail actually works better than the online version.

The main difference is speed. Regular Gmail is generally fairly quick, but you can still find yourself waiting at times for it to check in with Google’s servers. In offline mode or the very cool Flaky Connection Mode, everything — opening messages, searching for information, labeling missives — happens almost instantly, since all the data is local.

The tradeoff is that you don’t have access to all of your mail.

Google says, in essence, that it downloads your 10,000 most recent messages (they estimate that will cover several years for average users). But they also say that they identify your most important email threads and sync those. That’s a fascinating idea that’s not terribly well explained.

Here is what they say,

“We try to download your most recent conversations along with any conversations that seem to be important (regardless of their age). We also try not to dowload uninteresting conversations. This process is done heuristically and as with any heuristic can and will miss things. We’ll continue to tune things up, but more importantly, we’ll eventually provide a UI that will allow you to change the settings. Here’s a sketch of how these messages are selected:

Synchronization is based on the date of conversations. The system estimates a period of time to cover (at least 1 week in length) that results in approximately 10,000 messages being downloaded. For an average user, this means Gmail will end up downloading several years of mail.”