Halo 3's multiplayer beta will go live on 16th May at 1pm GMT and run until 7th June at 7.59am GMT, Microsoft said this morning.
You're promised a look at "new levels, weapons, vehicles and game types, as well as a chance to provide valuable data that [developer] Bungie will use in continued development".
The maps available will be Snowbound, High Ground and Valhalla, while you'll get to pootle around in the Mongoose all-terrain vehicle, using the new Assault Rifle, the Brute Spiker, the Spartan Laser and Spike Grenades to take one another out.
That's assuming you're on the beta. Gamers who secured a position by buying a specially marked copy of Crackdown, through the "Rule of Three" promotion or via another competition (or by ceaselessly begging friends at Bungie), will have no problems.
Microsoft reckons there should still be some copies of Crackdown on sale with a beta sticker on them, so keep an eye out.
In the meantime, why not take a look at a new behind-the-scenes trailer on Bungie's website or Xbox Live Marketplace? Called "Is Quisnam Protero Damno!", it offers a glimpse of Halo 3 multiplayer maps being made.