I have 3 GMail accounts I use regularly, so I just keep GMail open in three tabs, and use the GMail account switcher to switch each tab to one of the three accounts I care about.
Here's the official explanation (with this exact use case): http://gmailblog.blogspot.com/2010/08/access-two-gmail-accou...
Have you ever run into problems using it this way? Seems like this is somewhat outside of the normal usage (maybe because of all of the disclaimers when you add extra accounts?).