More money, more problems.
You can hear the warning cries all across the Internet. 'Malware is coming to the Mac!', 'Hackers are starting to target Apple products", "viruses are going to jump from your iPod to your computer", and so on. John Gruber of Daring Fireball has an excellent selection of every blog entry and news article predicting OS X disaster over the past few years. The point being that pundits have foretold doom and gloom for Mac for years without much coming true.
But, although OS X is unlikely to see a sudden storm of viruses and malware wash over its users like some sort of biblical plague, it won't stay clean for long. Just this week security researchers found a program called MacDefender that appears to be the first "fake antivirus" malware app for the Mac. Such a piece of malicious software wouldn't raise eyebrows on any Windows PC. But Macs, traditionally, are above such concerns.
And it gets worse. IT Research company CSIS Security Group reports that the first DIY malware toolkit has been created for OS X. It is currently being sold through underground forums. The "Weyland-Yutani BOT" (named after the evil megacorp from the Alien series) was designed to pull data from Firefox, Chrome and Safari. A version meant for Linux machines and the iPad is expected to launch soon. This has lead to renewed fears that some sort of worm/virus will target Macs by way of connected iOS devices.
When Mac market share was 4-5%, the platform stayed completely clean. It wasn't worth a hacker's time to develop something nasty for so few users. But now Macs control 15% of the US market share. And that number will only increase as the new iMacs spur sales ever forward.
No one likes having to deal with malware. But, in this case, its existence seems to be an inevitable sign of the platform's success. Mac is attracting malware because it has grown large enough to be worth messing with. It isn't a "good" thing or a "bad" thing. Just the only possible result of Apple's resurgence in personal computing.