When I heard the news that Gmail was offering "Multiple Inboxes" as part of Labs, the first thing that popped into my head was "finally--now I can check multiple accounts from the same place!" Unfortunately, that's not how it works.
Instead, Google's solution is simply to place the results from various filters and search queries off to the side of your main in-box. By default it sets you up with messages you've starred, and unsent messages from your drafts folder. This is nice and all, but you can hop to those two places from links in the left-hand navigation.
What I really wanted to do was to get in-boxes from two of my other Gmail accounts into my main Gmail account, and I managed to get it to work with a little tweaking. Here's how to do it:
Step 1: Enable Multiple Inboxes in labs. It looks like this:
Step 2: Log in to the additional account you want to view from your primary Gmail account and open up the settings menu.
Step 3: Go into Forwarding and POP/IMAP and set forwarding to on, and have it send a copy to your primary Gmail address. Repeat this with any accounts you want to forward.
Step 4: Go back into the settings menu of your primary Gmail account and open the Multiple Inboxes menu. In each of the panes simply type in "to:" followed by the e-mail address of one of the forwarding accounts. So it should look like "to:YourAccount@gmail.com" minus the quotation marks of course.
Step 5: At this point your multiple in-boxes are up and running, but you're still going to get these messages in your primary in-box too. To keep this from happening we're going to create a filter. Head back to settings in your master account, then click on filters. Create a new one. In the "to" field enter the e-mail address you're forwarding from, click next, then pick the "skip the in-box" option. Repeat with any additional forwarding e-mails and be sure to run it on messages that are already in your in-box to keep things nice and clean.
This system results in a few caveats that keep it from being a true "multiple in-box" experience. For one you'll need to actively set the right "send from" e-mail address when replying to a message from one of your other in-boxes. If you haven't done so already, this needs to be set up from the accounts menu in Gmail's settings.
Also it doesn't carry over any organizational goodness back to the source account, so if for example you star a forwarded message it's not going to have a star when you view it from the original account. The same thing goes for reading messages. So if you want to avoid an avalanche of unread mail back at the source accounts, one option is to set forwarded mail to be automatically archived. However, this might wreck havoc with your e-mail enjoyment on mobile devices and back at the source accounts. I recommend not turning it on, but if you're planning to only access your Gmail from the Web, and from your primary account this won't be an issue.
I'm hoping future versions of Multiple Inboxes will make this whole process a little easier to manage and simply let you plug in additional accounts. In the meantime this is a foolproof way to keep an eye on all three in the same place short of using a third-party Web mail aggregator like Fuser or Orgoo.
Update: As a few folks have pointed out there's another way to do this that avoids relying on forwarding and is smart enough to set the correct account when replying to a message. Here are the steps:
Step 1: Add your extra Gmail accounts to the "send e-mail as" option found in the accounts section in Gmail's settings. As mentioned before you'll need to confirm each account before you can start using it.
Step 2: Pull in the extra accounts you want to keep track of by adding them in the "get mail from other accounts" option in the accounts menu. When you're plugging in your account information be sure to select the "skip inbox" option as well as setting it to automatically label each piece of incoming mail. It should look like this:
Step 3: Go into the Multiple Inboxes settings menu and plug in "label:" then whatever label you selected in step 2.
Like the first method, there are a few hang-ups with this one. For one, slurping in additional accounts takes awhile after you first set it up. More importantly, it takes space in your primary Gmail account. If you're doing this with several large and active accounts you're going to start running out of space more quickly, which for most I'm guessing won't be a problem. Nonetheless it's worth keeping in mind.