By the time the doors of Apple’s (AAPL) flagship New York City store opened at 5 p.m. Friday, at least 330 loyal customers were lined up along Fifth Avenue and down 58th Street, nearly to Madison.

It was the first time anyone in the world could buy the iPad 3G, and these people wanted to be there for it.

First in line was Justin Sher, 31, an Apple specialist from Park Slope, Brooklyn. He works the night shift below the glass cube — the only Apple Store that never sleeps — and had logged plenty of time on the Wi-Fi iPads on display. He had proved to his satisfaction that the machine could do 10 hours of heavy duty game playing without heating up or running out of juice. But he was holding out for the model that can also run on AT&T’s (T) cellular network, and lining up outside his place of employment turned out to be the most efficient way to get it — even if it meant standing in the first-come-first-served queue for five hours.

The skies were clear, the sun was bright and it was hot on the plaza outside the store Friday afternoon. At 3:30 a pair of blue-shirted Apple employees appeared with two cases of smartwater to hydrate the crowd of more than 75 would-be customers. At 4 p.m., the store was emptied and the doors closed. By 4:20, the queue had swelled to 200. By 5:00 it had passed 330.

According to Piper Jaffray’s Gene Munster, there were 730 people lined up at the same location April 3 to buy the Wi-Fi iPad went it first went on sale, more than showed up for the launch of either the iPhone 3GS (350) or the iPhone 3G (540).

Apple had said it would make the 3G version of the iPad — which costs $130 more that the Wi-Fi-only model — available before the end of April, and it seemed determined to make good its promise with at least seven hours to spare.

It wasn’t clear ahead of time how many iPads would be on hand Friday. Supplies were sufficiently limited that according to Apple’s online store, units ordered that day would ship “by May 7.” By 5 p.m., the Fifth Avenue store had sold out of Wi-Fi-only models. The word in line was that the store had more than 500 3G iPads in stock — which looked to be sufficient unless everybody who showed up was planning to buy two.

Sales of the original iPad have been robust. Apple announced on April 14 that it had sold half a million in its first week. On Friday afternoon, Chitika Labs’ iPad counter showed more than 1,180,000 sold. But having been burned twice by that particular source, we’re waiting for Apple’s official count.

[Follow Philip Elmer-DeWitt on Twitter @philiped]