We'll see... there could always be exploits that work around the password thing.
There are those who believe Mac/Linux have no viruses because they're more secure and it'd always be that way, and there are those who believe it's only because the platforms are very unpopular. I like to think it's because they're more secure, but only time will tell us for sure if/when Mac/Linux become popular...
I do notice that the likes of Real Networks provide a really clean player that's almost a joy to use for OS X, but on the PC it's a spyware, adware, nagware horrible piece of scum that pops up "toast" every few minutes in the corner of your screen telling you about their latest exciting news. If ever the Mac became as popular, I bet they'd be one of the first to demonstrate just how annoying a piece of software can be even on the Mac without actually being a virus.