Yes, it's already happened. According to a post on examiner.com, AT&T is SOLD OUT of iPhone 4 pre-orders. Now, you won't be able to get the black iPhone 4 on launch day from there. I tried to log in there to see if I could verify it, but I couldn't even get into my AT&T account.
If you're going for the black one, it'd be a good idea to head to Apple.com and pre-order fast, because it's not going to last much longer at this rate. However, if you're waiting for the white one, this isn't really that big of a deal anyway.
Check out the whole post here.
Cheers, future white iPhoneers.