I know this will help me. I administer multiple Google Apps domains for different clients and during testing and configuration of each I am constantly logging out of one account and then back in to another.

From Mashable.com:

Now, according to Google Operating System, Google is testing a feature that will let users log in to multiple Google accounts in the same web browser. Personally, I have at least four different Gmail accounts open at all times, and this feature is very high on my wish list.

Google services that are currently being tested include Gmail, Reader, Sites, Calendar, Code and Docs. There is one limitation: if you turn on multiple sign-in, you cannot access Gmail in offline mode.