The talk among Apple (AAPL) watchers today is Toni Sacconaghi’s dogged pursuit of the 4 million iPhones Steve Jobs claimed to have sold as of Jan. 15, the date of his Macworld keynote speech.

AT&T (T), the iPhone’s exclusive U.S. carrier, reported yesterday that it had activated “just at or just slightly under 2 million” iPhones. That’s quite a discrepancy.

Sacconaghi, Sanford Bernstein’s Apple specialist, did the math and concluded in a report to clients that there are roughly 1.4 million iPhones “missing in action,” either unlocked or sitting in inventory. Assuming that 20% of those iPhones were purchased to be unlocked (a generous assumption given that a jailbreak for the latest iPhone firmware was only released yesterday), he believes that there are at least 650,000 gathering dust somewhere — in warehouses, perhaps, or in closets, as unwanted Christmas presents waiting to be returned.

Here’s how he gets that number:

• 3.75 million iPhones sold as of Dec. 29 (per Apple’s Q1 report)
• minus 2 million iPhones activated through AT&T as of Dec. 31 (per AT&T)
• minus 350,000 iPhones sold in Europe via O2, T-Mobile and Orange
• minus 750,000 iPhones purchased to be unlocked
• equals 650,000 unaccounted for.

Sacconaghi concludes:

This is negative in two ways: (1) it indicates end-user demand for iPhone is lower than many investors may think based on Apple’s sales figure; and (2) it points to slower iPhone sales in the current quarter, since much of this inventory is likely to be drawn down.

Of course, compared to other Apple analysts, Sacconaghi is something of a bear. One day before the Q1 earnings report and the subsequent run on Apple shares, he went out on a limb and predicted that the company would sell only 7 million iPhones in 2008. That’s considerably less than the 10 million target Steve Jobs set — a goal COO Tim Cook said on Tuesday he remained “very confident” they would hit.

Most Apple watchers shared Cook’s confidence, given the 4 million number Jobs had trotted out at Macworld. Today they’re singing a different tune.

“Apple might have a demand problem,” writes Tom Krazit at CNET.

Russell Shaw at ZDNet says the iPhone is at a “crossing the chasm” moment, stuck between early adopters and the mainsteam, and predicts that to survive its price will have to come down to $299 by the end of May at the latest.

Ewan McLeod at the U.K.’s SMS Text News waxes positively elegiac in a post entitled “The Apple iPhone will only ever be a bit player”:

This may be premature. A lot could change in the next 50 weeks. New apps. A 16 GB iPhone. A 3G model. New price points. New markets in Canada, Thailand, and maybe even China.

But one thing is certain: having promised and repromised to sell 10 million iPhones in 2008, there will be hell to pay in 2009 if Apple falls even a little bit short.