The maintenance to the Steam system is now complete.
When you start your Steam client you be automatically be updated to the latest version, which includes these major changes:

Offline Play

Account validation (via e-mail)

A new Half-Life 2 hardware survey

Support for purchasing games through Steam

There are a number of bug fixes in Steam included as well.
The Half-Life 2 survey will be updated throughout the day with the results tallied here.
Thank you for your patience during the downtime.