There's already a ton of such exploits. Most servers use Linux and many exploits of corporations this had to go through Linux (though many exploits aren't related to the OS at all -- eg, SQL injection is OS independent). I expect it's more common, though, that attacks on Linux systems are either meant to target servers or were personalized attacks that you're not gonna accidentally download.

On that vein, I also kinda suspect that many people who use Linux may be bigger targets for their employer than their personal PC. Which is actually scary, cause personalized attacks are far harder to defend against. I expect the average Linux user is technically savvy. Not a lot of money in try to do a standard, broad attack on such types (I think most attacks on personal computers are broad attempts that mostly depend on a small fraction of technologically incompetent people falling for simple schemes). But a personalized attack that happens to infiltrate a fortune 500 company? Now that's worth a lot of money. Using Linux won't protect you against those kinda attacks.