Malware encompasses a lot of stuff. I consider things like browser highjacks, redirects, etc (which is what you are describing) to be a different class from things like Crypto, FBI, root kits etc.
Browser related attacks, at this point, are still not drive by from what I have seen. It's a case of PEBKAC. This malware is generally very easy to remove via a full browser reset. Of course that will delete all of the stored info so that is a pain for some. At this point these are few in number and a good understanding of how OS X works will go far to understand how to manually remove them.
The true virus type material is a different animal. I've gotten my hands on two viruses and they were installed like any other program and easily removed. I know there have been several articles taking about some other exploits but they are totally devoid any real facts. Of course EU's can and will download and install bad programs without any research. I have yet to hear of any malware on OS X that is installed via a drive by. At this point the EU's are always making a choice to install something.
I'm sure as black hats turn more of their attention to OS X we will see more, especially of the browser related stuff. But at this point, given that OS X is *nix, finding infections via drive by's is highly unlikely. And if we do it will most likely be a zero day exploit so anti-malware programs will be useless anyways.