If you have more than one Gmail account you will now be able to switch between them without logging out and back in. The new multi-account sign in feature was launched yesterday and is available on most of the popular Google apps. You can sign into 3 accounts maximum and whatever account you're in when you enable the multi account feature will become your default account.
If you don't see this feature yet it will show up in your account settings in the next couple of days. I don't see it on mine yet
Clicky for more details.