Google is testing out a new feature for some users which will allow them to sign in to multiple Google accounts at once, as opposed to having to run multiple browser sessions for separate accounts. The feature presently only works for Gmail, Google Calendar, Google Reader, Google Docs, Google Sites, and Googe Code, according to Google Operating System.
Essentially, the service will allow you to open two (or more) Gmail accounts in separate tabs, as an example. However, it should be noted that the Gmail offline mode has automatically disabled when multiple account sign-in is enabled.
No information has been released with regard to how long this will be tested and when/if it will be launched publicly.