Got my iPhone Friday night, iTunes wouldn't accept my porting number, so i registered with another number. Went into AT&T on Saturday around noon, talked to the guy there, he got it squared away and said it could take up to 24 hours. Called the number he gave me Monday around 10 when it still hadn't happened, talked to a lady there who finished it up for me. Restarted the iPhone and it worked!
One thing she said was that the old phone had to be turned off, and then you had to turn off and back on the iPhone to get it to work... there was about a minute delay before the number switched after i did this, during which i thought she was lying to me