Valve's just announced Steam Guard system is now in beta. Word is: "As a Steam account holder, you can now take advantage of this additional level of account security, further prohibiting others from gaining access to your account. As a beta participant, once you've verified your email address with Steam, Steam Guard becomes available for your use and is enabled for your Steam account by default." To demonstrate the security this provides, Gabe Newell gives out his username and password at Cebit (thanks VG247 via Reddit).