Alex Chitu at Googler Operating System has spotted a new Google feature that should please a lot of users. Some accounts now have an option for multiple sign-in, meaning they can log in to more than one Google account from a single browser at the same time.
This is likely more than just a test, because Google has information about the feature in its Help Center. It is probably just a matter of time until all users have the option.
At this point, it looks like the only Google products that support the feature are App Engine, Google Code, Google Calendar, Gmail, Google Reader, Google Sites, and Google Voice, with Google Docs support coming soon. Other products will simply default to the first account you signed into from the browser.
Multiple sign-in will affect any offline activity you have connected to a Google account. Google offers the following warning:
Enabling multiple sign-in will disable Offline products like Offline Gmail and Offline Calendar, as well as any browser bookmarks you've set to link to your accounts. If you use Offline Gmail, make sure to sync your offline mail before enabling multiple sign-in so you don't lose any messages in your outbox. If you would like to continue using Offline Gmail, Offline Calendar, and browser bookmarks linked to your accounts, do not enable the multiple sign-in option. If you have already enabled multiple sign-in, you may disable it.
I'm only speculating here, but this feature could play a significant role in Google's rumored "Google Me" social network initiative. Being able to easily operate multiple accounts would make it easier for users to manage multiple business-related accounts, while keeping them separate from personal accounts - like being able to control your business' Facebook Page while being logged in as yourself. Google also recently announced that all YouTube users would be required to have their accounts tied to Google accounts.
Google has yet to officially announce the multiple sign-in feature, but we can probably expect that they will do so shortly. More info is available here.