Google just enabled multiple simultaneous account sign-ins. This should greatly facilitate its use for those many of us who maintain more than one Google account, for instance with a corporate and a personal account. There are, however, significant limitations at this time, which may cause you to decide that you should wait.
First let us see how to use it - you have to turn it on first. Go to www.google.com/accounts and select the Edit link under Multiple Accounts. You will then have to check several boxes, to show that you understand the limitations of the feature, before turning it on.
At this stage, the feature's scope is VERY limited. It works for your general accounts, for Gmail, Reader, Calendar, Voice, Code and Sites. It does not work if you use most other Google services (including Blogger), or if you use Offline features. Even with Gmail, there is a risk that you will lose unsent mail. And - it does not work for mobile use. Whatever account you log into first in the day becomes your primary account, and it is this account into which you will be logged in for ancillary services that do not support multiple accounts yet.
Clearly, Google is planning to progressively generalize this capability to the rest of its infrastructure. We can't wait! Until then, be sure to clearly understand its limitations before turning it on. For heavy users of non-supported services, it may be best to wait further.
Want to read more about it? Try lifehacker, CNET, mashable, geek, SF Chronicle, Information Week, Google Operating System blog (unofficial), or the Washington Post.