The slowly increasing popularity of OS X could, at one point or another, attract greater attention from would-be criminals and those bent on breaking into peoples systems. While Windows is the target platform of choice for most, OS X has come under increasing levels of attack in recent times. That's what security specialists are noticing about both iPhones and Mac machines, particularly this year.
Not all of the increased attention is bad with increased popularity also came a rise in the people reverse engineering Apple protocols and protection systems, which led to wonderful things like the ability to unlock your iPhone in user-friendly ways. Ultimately, though, it also means that there will need to be an increased vigilance on Apple's part to make sure systems stay patched, as well as an increasing number of malware created for it, such as a new trojan spotted earlier this year.
So far Apple has been doing well on releasing regular updates but as Windows has proven before, once you reach a critical mass of users it will be very hard to issue a patch before many become compromised.