When Apple beat Wall Street expectations convincingly on Monday after its best quarter ever, its share price fell.

Financial analysts were worried about Apple's once-stellar profit margins, the health of its irreplaceable chief executive Steve Jobs and fears that a slowing global economy will mean weaker sales of iPods, iPhones and MacBooks.

Amid this uncertainty, a different type of analyst told me of another troubling development for Apple, one that is probably not yet written into any financial models: Apple is beginning to attract the attention of hackers and malware writers.

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A big factor in Apple's success in selling 2.5 million computers last quarter is growing user disaffection with Windows. Everything from recurring Vista headaches to security fears are driving Windows users into the Mac camp.

Ironically, the resulting Mac sales are coinciding with — and causing — a new upsurge in malware written specifically for Apple users.

"It's still a drop in the ocean compared to Windows vulnerabilities, but [Mac vulnerabilities] have become more sophisticated and more criminally minded, rather than just proof of concept," Graham Cluley, senior technology consultant at the security company Sophos, says.

Sophos reported Wednesday that two new Mac-based Trojans that emerged in February and June ought to shake Mac users of their misconceptions that their computers (and, eventually, iPods and iPhones) are impenetrable.

To put this in perspective, the first really pernicious piece of Mac malware emerged only in October 2007, Cluley says, suggesting that a worrisome trend is about to get worse.

• Click here to read the rest of this story in the Times of London.