-You will not get your email until ~2 hours after your account gets flagged
-The box art screen will never show Diablo III beta accounts
-Some people with flagged accounts are unable to log in to the game right now
-You will usually not be able to post in the Beta forums until you log out, close your browser, re-launch it, and re-login.
Up at the top of your screen you will see "ACCOUNT" in capital blue letters. This page will immediately display to you whether or not your account has been flagged for beta access.
Refresh this page as many times as you see fit and good luck.
You will see this if you get in:
Some additional information from a blue in regards to beta invites:
...[we] will do waves of invites as needed. When I say "as needed," I mean when the servers are stable and we're comfortable with the room we have to get more people in the game. We won't do waves of invites if we feel our infrastructure at any given time can't handle an increased concurrency.
...[beta invites] will typically go out during standard business hours (9 a.m. - 6 p.m. Monday - Friday, North American Pacific time).
Note that he said "typically". The earliest I have seen accounts get flagged thus far is Noon PST (first press invites) and the latest I have seen accounts flagged is 8PM (first F&F invites).
Work-in-progress Opt-In system specifications database: http://us.battle.net/d3/en/forum/topic/3279038123?page=1