If you don't have your copy yet but want to save yourself some time later you can download the client. Here is a list of exe links.
Remember to uninstall the beta if you ever downloaded it. More on that here.
Also there will be a Starter Edition that you can download and have excess to when the servers go live. You must get a guest pass from someone to be able to play the starter edition. Guest passes can be found in the retail box of both CE and normal versions.
Diablo III Starter Edition provides players the ability to try out a limited version of Diablo III. Guest Pass keys allow a player to give friends the opportunity to play the Starter Edition of Diablo III. Guest Pass keys are included in all box copies of Diablo III.
Players on Starter Edition game licenses have the following restrictions:
Act 1 up to the Skeleton King is availble.
Level 13 cap
Matchmaking only with other starter edition players.
No Real Money Action House.
How do I get the Diablo III Starter Edition?
Log into your Battle.net Account Management page and navigate to the Download Game Clients section. The Diablo III Starter Edition will be available for download there.
Players without a Battle.net account can create one, and then download the Starter Edition.
Note: The Diablo III Starter Edition will not be available without a Guest Pass until 30 days after the game launches.
How do I download the Diablo III Starter Edition?
Navigate to the Download Game Clients section of your Battle.net Account Management page and click the icon for your operating system under the Diablo III heading.
How do I upgrade from Starter Edition to the full version of Diablo III?
Log into Battle.net Account Management and select your copy of Diablo III Starter Edition on the Game Account list. On your Diablo III Starter Edition Game Management page, you will see your upgrade options on the right-hand side.
So as long as you know someone that is getting the game at midnight, or already has the game, you can get the guest pass from them and play at least up to lvl 13. As of when I wrote this the Starter Edition wasn't up for download, but I would check from time to time to see if you can get it through out the day. Not sure if you can use the full client download (like the one I posted the link to above) and still play the starter edition. I will update when I see the starter edition up.