At high noon (well, Pacific Time high noon), the "beta" section of Charlie Oscar Delta will become active and invites to the Call of Duty 4 beta will be handed out on a first come first serve basis.

To get your public beta you need to first register, then, at noon, go to the "beta" section of the site. At that point, the "enter now" button will actually work. You'll have to answer three CoD trivia questions and also hope that their server doesn't crash.

Good luck!