AT&T sent out emails to customers today which reveals that they are limiting iPhone sales to "one per person":
With iPhone arriving June 29th at 6 p.m. in AT&T retail stores, the wait is almost over. Use our online store locator to find the closest AT&T store that sells Apple iPhone (limit one per person). We'll show you maps, directions, and everything you need to get your hands on an iPhone as quickly as possible.
There's been no official word if Apple will be similarly limiting their sales, but it may be a safe bet. Appleinsider reports that Apple Stores are preparing by requiring some employees to stay overnight on Thursday night to prepare for Friday's launch.
Finally, we've heard that some above-average size Apple Stores may be getting on the order of 1000 iPhones for Friday's launch, with a ratio of inventory being heavily skewed towards 8GB iPhones over 4GB iPhones at approximately a 4:1 ratio.
MacRumors forum members are meeting up at their respective locations to stand in line.