Earlier this week, it was discovered that Google was letting users sign in to multiple accounts in the same browser, a feature Google has always lacked, that many users have been pining for.
Now Google has officially announced the feature, via the Gmail blog. Along with that announcement comes a few warnings. Google says to keep the following things in mind, as the multiple sign-in feature is intended for "advanced users":
1) Not all Google services support multiple account sign-in yet. For the services that don't support it (like Blogger and Picasa Web Albums), you'll be defaulted to the first account you signed in with during that browser session. So if you click a link from Gmail to Blogger, for example, you'll be logged into Blogger with the first account you signed in with, even if you clicked the link to Blogger from your second Gmail account.
2) We're still working on making Gmail and Calendar work offline with multiple sign-in. If you rely on offline access, you probably don't want to enable this feature quite yet.
3) Multiple account sign-in only works on desktop browsers for now, so if you use Gmail on your phone's browser you won't see this option yet.
You can switch to multiple sign-in if you go to google.com/accounts.