Why?

Apple stock took a deep dive in early trading Wednesday.

Among the troubling signs in Apple’s earnings were indications that U.S. iPod sales slowed an unusual amount in December. Courtesy: Apple

Apple’s new iPod lineup (Photos 1-5) Flash storage and more in HP’s redesigned laptops (Photos 1/6) Seagate’s slick storage for Mac and PC (Photos 1/9)

To some extent, it’s a case of one bad Apple (AAPL) spoiling the bunch. Steve Jobs & Co. is seen as the most innovative, growth-producing group in tech. And if the U.S. consumer’s economic troubles are starting to rattle mighty Apple, high fliers like Research In Motion (RIMM) and Google (GOOG) might not be immune, either.

Indeed, Apple’s holiday performance showed signs that the company’s not unstoppable in 2008. In particular, Apple’s cautious outlook, weakness in U.S. iPod growth and the unpredictability of iPhone sales left Wall Street’s pessimists plenty of reason to doubt. And in this jittery market, those pessimists have a lot of power.

First, a recap of Apple’s good news — and there was plenty of it. Apple turned in revenue of $9.6 billion and profit of $1.6 billion for the holiday quarter, blowing past the average analyst estimate. The company shipped a record 2.3 million Intel (INTC)-based Macs during the period, and actually sold as many iPhones as computers. In the process Apple generated $2.7 billion in cash, bringing its war chest to $18.4 billion.

But there was troubling news, too. On the conference call with analysts, Chief Financial Officer Peter Oppenheimer admitted that iPod sales merely met the company’s expectations, rather than exceeding them. Part of the reason, he said, was that U.S. iPod sales weakened in December — it took overseas sales to make up the difference. “In the U.S., in the gift-buying season, we saw a slightly different curve,” he said. “That was made up for in our very, very good growth internationally.”

There was more. It was clear that Apple executives weren’t sure what to make of the iPod slowdown. Maybe it’s the U.S. economy. Maybe the presence of the higher-priced iPod touch convinced people to save up and buy one iPod instead of two, they suggested. And then there’s the possibility that the iPhone is starting to eat into iPod sales. “In the U.S., where iPod unit sales were flat year over year, it could have been one of the factors, but other factors played into that as well, so it is very difficult to say with any precision whether there was cannibalization or not,” said Chief Operating Officer Tim Cook. Cannibalization would be a bad thing. It would mean that iPhone growth doesn’t purely add to Apple’s results — it also takes away from the iPod.

Add in the fact that iPhone sales are so hard to predict (this is Apple’s first time selling them in a March quarter), and Apple’s outlook made Wall Street nervous. For this current quarter that will end in March, executives promised revenue of $6.8 billion and earnings of about $850 million. And while ordinarily analysts would take that number with a wink and expect Apple to easily beat it, this year they’re not so sure Apple can.

So what does this mean for the rest of the tech industry?

It means Thursday will be a particularly important day, as number-one phonemaker Nokia (NOK) and software giant Microsoft (MSFT) report earnings. Unlike Apple, Nokia’s phone empire is weak in the United States and strong in Europe and Asia; so an encouraging report and a solid outlook from Nokia could reassure investors that Apple’s international iPhone plans can work. Microsoft meanwhile can give an overview of consumer and business spending patterns. If businesses continue their trend of planning to upgrade to Windows Vista in large numbers, it may signal that the economic environment isn’t horribly bad.

But until then, tech stocks seem likely to be driven by the sour news from Apple and Motorola (MOT), and whatever signals come out of eBay (EBAY), Qualcomm (QCOM) and Symantec (SYMC).

It’s a little ugly out there.