Valve have announced that Steam Guard is now out of beta and available to all:

* I can confirm Steam Guard was enabled by default on my account. {LINK ENTFERNT - Bitte kein Spam. Nehmen Sie sich vor Betrug in Acht.}http://bit.ly/fEP3uX <-- How about yours?

As a Steam account holder, you can now take advantage of Steam Guard, a new feature of Steam that enables an additional level of account security. Your account may already be protected by Steam Guard, as the new feature automatically begins protecting your account once you've verified your email address with Steam.

With Steam Guard enabled, anyone attempting to login as you from an unrecognized computer or browser must first provide additional, one-time authorisation.

A special access code will be sent to your contact email address, and this code must be entered into Steam before your first login on an unfamiliar computer is complete. You will also be notified if any login attempts from computers other than those you've authorized occur. Steam Guard essentially acts as a form of "User Rights Management", where you as the user have greater control over access to your stuff.

Their knowledge base has some more information.

You will need to update your Steam Client in order to take advantage of this. To check if you have Steam Guard available, click Steam -> Settings, and under account, a new field should appear as "Security Status". A new button, "Manage Steam Guard Account Security..." is also available to enable/disable/deauthorise other machines (Otherwise, simply "Check for Steam Client updates...".

Right now, Steam Guard is limited to emailing a security code to authorise a machine, but future versions will offer other methods of authentication (such as the Intel® Identity Protection Technology).