Google is testing multiple account logins for some users according Google Operating System (unofficial blog). The option shows up on testers’ Google Accounts page and works on Gmail, Google Calendar, Google Reader, Google Docs, Google Sites and Google Code.
If this rolls out to everyone, it will be a very welcome feature for many, as currently, Google users that want to have more than one account running on their computer at a time need to have two browsers running simultaneously. If you are a tester, however, and you turn this option on, you won’t be able to use Gmail in offline mode, which may be a disadvantage for some. That said, hopefully this will be a feature that Google can put in the fast-lane to get out to all users as soon as possible.