If you have got the news about the Diablo 3 open beta weekend (a.k.a. stress test), you have probably started downloading the beta client already.
The problem is once you have the download complete – Blizzard explicitly stated you need to wait to 100% and not click play before that – you will probably not be able to log in. Welcome to error message 3003 (login problems) and error 37 (servers busy).
Even Diablo’s technical forum is filled with complaints about the 3003 error. It seems that the forum servers themselves are running pretty slow and posting is next to impossible.
Update: Since only the EU servers are up and running, make sure you select EUROPE! Go to Options > account > server region selection > europe.
Update 2: Actually, the US servers are now up so go ahead and try them instead. I just managed to log in and create a barbarian! Go to Options > account > server region selection > america.