His conclusion: most of the devices he describes as “missing in action” are not sitting in warehouses, as he originally surmised, but were siphoned off into the gray market for unlocked iPhones. His best guess is that in 2007 as many as 1 million iPhones may have been hacked by resellers and activated by carriers that are not paying Apple a kickback on every monthly charge.

This is a big problem for Apple, says Sacconaghi. For every 1 million iPhones that get sold for unlocking, the company forgoes, by his calculation, $300 to $500 million in future revenues and profits.

Here’s the dilemma as he sees it:

If Apple were to somehow stop the sale of unlocked iPhones (by forcing customers to activate them at the point of purchase, say) the company might miss its target of selling 10 million iPhones in 2008 — and forgo even more revenue and profit.

But if Apple does nothing, it gets hit with a double whammy. Not only are its healthy gross margins reduced (unlocked iPhones generate 50% less revenue for Apple and 70-75% less profit, according to Sacconaghi), but growing new markets overseas gets harder. If the company can’t stop the flow of unlocked iPhones into a country like China, what’s the incentive for a Chinese carrier to pay the stiff premium Apple demands for the right to be that country’s exclusive carrier?

Sacconaghi continues to believe — almost alone among major analysts — that Apple will have a hard time reaching its goal of selling 10 million iPhones in 2008.

“In order to achieve this target,” he writes, “we expect Apple will have to lower the iPhone’s price, introduce new (likely lower-end) models, and/or forego revenue-sharing in certain geographies, all of which would compromise the iPhone’s economics.”

Of course, it’s widely expected that Apple will lower the price and introduce new models this year. It remains to be seen whether it will have to do anything about those sweet revenue-sharing deals it’s been cutting with the carriers.