[Jeremy's take on Frank's take on Mac vulnerabilities...]
OK, sort of...
There have been plenty of security vulnerabilities exposed in UNIX, which is what is behind OS X. Just not widely publicized.
Both unix and linux have a "file" command, which can (usually) identify a file type, regardless of its extension. It's virtually certain that this command is on OS X. You can get to a shell prompt and type 'man file' and you should get the manual page for the command.
Personally, I like file extensions. They allow you to see at a glance what type of file you're dealing with, without costing anything but a few keystrokes. But then again, I've been doing this stuff since long before there was such a thing as a mouse.
My pet theory is that the reason you don't see many mac or unix related viruses is, indeed because of market share. However, this (in my theory) is because the viruses are being written and distributed by (or for the benefit of) the anti virus software merchants. I understand that there are vandals, but look at who is profiting from all this. Not Microsoft, not Apple, not you, not me, not the guy who wrote the virus (if my theory is incorrect), but the anti virus companies are making a fortune.  For one of those companies to produce a new virus and distribute it widely would be easy, and tantamount to printing their own money.
Call me crazy, but that's what I think.
Remember to make lots of backups,