Today the thing slamming the stock (chart left) is an unconfirmed rumor that the company is cutting back iPhone production “from 9 million units to 4.5 million units,” according to a note from trading firm Miller Tabak & Co. (Miller Tabak has since said it issued no such note – TheStreet.com attributed the note to the firm.)

The rumor is pretty ridiculous for several reasons. Just a few of them:

1. Projections. Apple has publicly stated that it plans to sell its first 1 million iPhones by the end of September. Apple has also said it plans to sell 10 million iPhones in 2008. The company reiterated its iPhone projections in its earnings less than a week ago, so it would be unusual for plans to change this quickly.

2. Manufacturing. Because Apple has not yet finalized deals with carriers in Europe and Asia, it seems unlikely that it has final plans for exactly where and how 9.5 million iPhones will be manufactured. (Because the iPhone has not even taken a bow in Europe, it would be very difficult to forecast demand or sales rates.) Also, assuming the Miller Tabak & Co. rumor isn’t completely made up, it could be that Apple has simply hedged its bets by having more than one company manufacture iPhones.

3. History. The market now has a habit of pushing Apple stock up or down based on rumor and innuendo. Analysts say all kinds of things about Apple, seemingly just because it’s a popular thing to do and there are few consequences for being wrong. Remember the folks who guessed that Apple and AT&T (T) sold more than 500,000 iPhones in the first 30 hours? (The real number was 270,000.)

Of course, a big part of the reason why rumors can drive Apple stock so often is that the company says very little about its plans. Last week’s quarterly conference call with analysts sounded like a White House press conference, what with all the secrecy and unanswered questions. Apple executives wouldn’t tell Wall Street what kinds of iPhone payments the company gets from AT&T, how many Apple sold vs. AT&T, how many Apple TVs are selling, or any number of other details.

Analysts who cover Apple are clearly used to the drill with the tight-lipped company. Often the bulk of Apple’s own PR and investor relations staff don’t even know details about the company’s new products until the moment CEO Steve Jobs announces them.

The reality is that Apple is run on a need-to-know basis, which adds oomph to its product announcements, and also breeds a lucrative rumor community. Today, those rumors are shaving about $4 billion in market cap off of Apple stock.