Fasten your seat belts.

Although Apple should report better-than-expected quarterly earnings after the close Monday — it almost always does — its shares could be in for a bumpy ride on Wall Street.

Apple’s (AAPL) stock price — having bungee-jumped from $200 in late December to below $120 in mid-March and then back up to $190 in mid-May — has been drifting lower ever since, despite the high-profile launch of a new iPhone and the expectation of sharply higher earnings.

According to Thomson Financial’s survey of analysts, Apple is expected to report net income of $972.6 million, or $1.08 per share, on sales of $7.4 billion. In the same period last year the company earned $818 million, or 92 cents a share, on sales of $5.4 billion.

But these days, even 18% earnings growth from Apple is unlikely to impress the Street. The company could report its best third fiscal quarter (our calendar Q2) yet and still lose market value.

By most accounts, Q3 was a strong one for Apple. In a report to clients issued Friday morning, Piper Jaffray’s Gene Munster saw good news in all three of its key divisions:

• Macintosh: He believes Apple will announce quarterly sales of 2.35 million Macs — 33% year-to-year growth in an industry that is growing at half that rate. (On Wednesday Gartner reported that Apple is now the No. 3 computer maker in the United States. See here.)
• iPhone: Munster is expecting Apple to report that it sold 730,000 iPhones in Q3 — slightly better than the 700,000 Apple already reported. (The 1 million iPhones that Apple claims flew off the shelves in three days last week don’t count until next quarter.)
• iPod: Although many had predicted that the iPhone would cut into iPod sales, Munster is seeing little cannibalization so far. He expects Apple to report 10.5 to 11 million iPods sold — up from his previous estimate of 10.25 million.

But Wall Street’s antennae are finely tuned for disappointment. Although Apple just had a record-breaking, made-for-TV product launch — with people still queuing up for the iPhone 3G a week after it went on sale — none of that produced much traction in Apple’s share price. Investors seemed to concentrate instead on the fact that Apple’s iPhone activation servers melted down, that most of their stores ran out of product and that the new MobileMe suite of Web services is a mess that the company still hasn’t cleaned up (see here).

Shaw Wu, the top Apple analyst at American Technology Research, is focused on the company’s gross margins, which came in surprisingly low last quarter for reasons that were never adequately explained. He’s expecting gross margins of 33.5%, slightly higher than the company’s 33% guidance. But he notes that lower component prices last quarter did not translate into higher gross margins. “Investors chose to ignore this and gave AAPL a ‘free pass,'” he writes. “Given the macro environment, this quarter investors may not be so forgiving.”

In an article entitled “Why I’m Shorting Apple Ahead of Earnings” that got a lot of attention on the Seeking Alpha website last week, investor Ben Shuleva ticked off a litany of reasons he expects Apple’s share price to get punished after Monday’s earnings report, from cutbacks in education budgets that could eat into Apple’s back-to-school sales to the way Apple books iPhone revenues over 24 months, an accounting complexity the Street still doesn’t understand, no matter how many times Apple explains it.

“I am not bearish on Apple long-term,” Shuleva wrote. But… “I am willing to make a significant bet that on a short-term basis, Apple’s share price will deteriorate.” (link)

Finally, there’s the matter of Apple’s guidance for its fourth quarter, which ends in September. Peter Oppenheimer, Apple’s chief financial officer, has been known to send the stock into a tailspin by issuing numbers that are miles below Wall Street’s expectations. “We believe AAPL will likely continue its tradition of conservative guidance,” writes Wu, with considerable understatement.

The question is, how conservative? If it’s the usual 9% or 10% below expectations, it shouldn’t make much difference. Anything lower could damage the stock. And if Oppenheimer offers guidance that’s better than expected, who knows, the stock might actually go up.

Apple executives will discuss the company’s Q3 quarterly result and offer guidance for Q4 in a conference call Monday at 5 p.m. EDT (2 p.m. PDT). Apple plans to webcast the call (click here). We’ll be dialing in — and live blogging — at Apple 2.0.