I've heard that the process has changed slightly this year, so someone else may want to chime in with some clarification.
But bottom line: if you're a) very, VERY well-qualified (to the point that the AFA thinks they're going to lose you to some Ivy early acceptance gig) or b) a heavily recruited athlete -- yes, you may get a letter before March to tell you that you will get in if you get a nomination.
If that doesn't happen, but your member of Congress gives out principle nominations, you get it, and you meet all the admission requirements, then you know you get in. (Principle nomination = member of Congress saying 'Assuming he/she is qualified, I 100% want this person to have a slot.')
Otherwise, you get to be with the rest of the crowd, compulsively comparing SAT scores on the Internet. I know--thrilling.