But there have been as noted above. The new vista requires users to say yes when running as admin as well; however, I bet the excuse antirepublicrat uses won't be acceptable for Microsoft when idiot users click on a link and say yes to allow admin access and wammo virus hits. The fact that most machines will be windows it's more likely that a virus writer will gamble that even if 1% of the users are dumb enough to say yes...they'll catch hundreds of thousands if not millions of users with their virus. If you catch 1% of the mac users that say yes to an admin access request...then you have caught hundreds of people. Not exactly newsworthy.
But those viruses were for Mac OS 9 and earlier operating systems that were discontinued years ago. There have been zero viruses in the wild for Mac OS X since it was introduced five years ago.
The vast majority of Mac OS X users don't even have anti-virus software. In the Windows world, they would be sitting ducks.
For trojans of this type, it'll still be the user's fault. However, Microsoft's implementation has its own problems in that it makes that box pop up so much in normal usage that users will get used to simply typing in the password to keep working with their normal stuff, and they may not notice that they just allowed something they shouldn't have.
And please learn something about the platform you denigrate before posting about it. There is less relation between OS 9 and OS X than between Windows 3.1 and Vista.
As also noted above, no, there haven't been. Mere proofs-of concept demos that were never spread in the wild. They have NOT been seen outside of a file sent to (created in?) a security company who then promptly reported it with an offer to sell AV software.
A virus without a vector is impotent.