As of 6:00 a.m. EDT Tuesday, all three models (8GB black, 16 GB black or white) of the hot-selling device were sold out in 21 states, according to Jim Neal, a retired PR man living near Kansas City who took the time to check each of Apple’s 188 U.S. retail stores using the company’s iPhone availability widget.

“All told,” he writes, “117 Apple stores reported having sold out of all models and only 27 stores indicated they had all three models in stock.”

The states reporting no stock on hand were Connecticut, Delaware, Hawaii, Indiana, Iowa, Kentucky, Louisiana, Maryland, Michigan, Minnesota, Missouri, Nebraska, New Hampshire, New Jersey, New Mexico, Oregon, Rhode Island, Tennessee, Utah, Washington, Wisconsin.

Apple (AAPL) advises shoppers to check availability after 9:00 p.m. the night before they visit an Apple retail store — and, if necessary, get up at the crack of dawn.

“Shipments arrive most days,” according to the site, “but be sure to arrive early since iPhone 3G is sold on a first come, first served basis.”

1,800 AT&T (T) stores also carried iPhone 3Gs, but they were “virtually” sold out the first day of sales, according to a company spokesman, and there is no word on when they will be restocked.

UPDATE: Jim Neal did another survey Wednesday morning. As of 7:00 a.m. EDT, 146 Apple stores in 23 states were completely out of stock.