While Apple's install base is steadily growing, I still think they have a significant advantage in obscurity that grants them security.

i.e. Why do virus-writers target Windows? Sure, it has the largest install base, but Windows also runs a lot of servers and embedded systems. These two things are the real jackpots. What's taking control of one person's computer compared to taking control of a server with credit card information or a system running a billboard in Times square? Apple doesn't do embedded systems and their servers, while some do exist, are exceedingly rare.

Is security through obscurity enough though? That depends on who you are and how interesting your data is.