"I think it's demand," said Brian White of Topeka Capital Markets, of the quick sell-out. "Apple wouldn't want to upset customers by running out in just an hour."

There are supply constraints, said White, who noted that they're inherent in any mobile product launch to one degree or another, calling them "part of the game."

"Cook said [Wednesday] that this is the fastest launch in Apple's history, with nine countries rather than seven [in the first wave]," said White. "He wouldn't have planned such an aggressive launch if he thought that supply would be a material event."

If Apple had been overly concerned about supply issues, argued White, it would have reduced the number of opening markets from nine to, say, five. It didn't.

Before Apple's Wednesday unveiling of the iPhone 5, White had forecast 5 million to 5.5 million units pre-sold worldwide over the first weekend, which included today, tomorrow and Sunday. White had pegged opening-day sales of 1.3 to 1.5 million.