Another day, another set of launch-day photos, this time outside AT&T (T) outlets across the U.S.

Having been overwhelmed with pre-orders for the iPhone 4, Apple’s U.S. carrier had to postpone walk-in sales until June 29.

And sure enough, when AT&T stores opened Tuesday morning across the country, there were lines of customers waiting outside.

CNET reports that one New York City outlet had run out of 16 GB iPhones within 20 minutes and announced 10 minutes later that there weren’t enough 32 GB models left for the 60 people still waiting in line. Names were taken, but no promises made.

We’re hearing similar stories from Las Vegas, NV, Lexington, KY, Boulder, CO, and Athens, OH.

Apple (AAPL), meanwhile, has pushed the ship date for iPhone 4s ordered online back three weeks.

In a note to clients issued Wednesday, Kaufman Bros.’s Shaw Wu estimated that if it weren’t for production constraints, Apple might have sold as many as 2.5 million iPhone 4s in the first three days, rather than the 1.7 million it did sell.

See also:

• 1.7 million iPhone 4s sold in 3 days
• iPhone 4 stockouts: 60% of Apple Stores, 100% of the rest
• What’s driving iPhone 4 sales
• What the iPhone means to Apple

[Follow Philip Elmer-DeWitt on Twitter @philiped]