On March 6th AdCenter will hold an "open enrollment" for people wishing to participate in the AdCenter pilot. You'll be able to sign up and get instant access to your account - no waiting for an e-mail invitation.

Yes, the signup from has been live since last year. When you sign up though you'll be put on a list and sent an e-mail at some later date that will allow you to open your account. If you signup on the 6th during that 3 hour window you won't have to wait for an invite e-mail.

AdCenter is for advertisers - i.e. it's the AdWords competitor.

The source is the AdCenter team via e-mail. It hasn't been posted anywhere else that I am aware of. The official post will be made on the AdCenter blog tomorrow.