At this point, almost all Google Apps domains have been transitioned over to the new account infrastructure. As you may recall, this was done in order for Google Apps for Business customers to enjoy some of the benefits that regular Google Account users enjoy.
One of the many things that have changed due to the transition process is multiple sign in. If you enjoyed this feature whereby you could be in more than one account in one browser session in the past, you'll need to go enable it for individual accounts.
In order to do this, you'll need to be signed in to a Google Apps instance and head over to the multiple sign in settings page. From here you'll likely see that your ability to do this is "Off" by default, and what you'll need to do is turn this "On".
Yet there is a caveat to this: for users who enjoy the usage of offline functionality because they are mobile or otherwise cannot get consistent internet connections, enabling multiple sign in may not be ideal. As Google states:
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.
You'll know that your offline mail is syncing when you have offline enabled and you see this tiny circular icon in the upper right of your Gmail inbox.
In the end, if you are not using Offline Gmail, having multiple sign in implemented is really helpful. Most people have more than one Google account, and because syncing between accounts is problematic and potentially troublesome, this is a great way to access all data yet still have some divided lines. Once you have it enabled, you can go up to the top right where your account information is and select "Switch Account" to sign in to another session.
Select "Sign in to another account".
After the additional sign in, you can go back to "Switch Account" in the top right, and you'll see multiple accounts available to you that you can now switch back and forth from.
Have you enabled multiple sign in for your Google Apps account? How do you like it so far?