They don't support such a feature at the moment. Note that E Cubed's link is from Google Operating System, which is not official and is not affiliated with Google, Inc. It wouldn't surprise me if Google were to release some sort of feature to allow signing into multiple accounts.
However, not only is E's answer from an unofficial source, but it doesn't sound like what you're asking about. You're asking about something more like linked Windows Live IDs, where you can sign in once and switch between accounts using a menu. Google doesn't support that and I haven't seen anything to indicate that such a feature is in the works (though I'm sure a lot of people would love it).
In the mean time, the best you can do is to export whatever you need from alternative accounts and import the information into the corresponding services in the account you would like to use as your primary login. It's not ideal by any stretch of the imagination, but it's the most you can do with Google's current Accounts feature set.