Note that this would also "solve" the problem of virtually all active Windows malware. What Mac users have failed to understand over and over again is that the vast majority of Windows malware depends on a user running an application, rather than on a specific Windows security flaw. Mac OS X is no more secure from this than Windows: the only additional protection is gives you is that it requests a password if you want to do anything that requires admin privs. You can create malware that causes all sorts of mayhem without admin privs.
This is great, as far as it goes. However, it doesn't mean you have some kind of uber-OS that's completely protected from the determined malware writer. First of all, lots of people will happily download stuff from (ahem) "not entirely legal sources" that look like installers for applications. They'll then happily put their admin password into the resulting "installer" and, when it doesn't work, put it down to dodgy software - not knowing they've just installed something much more malicious. There's nothing in Mac OS X that prevents a virus writer from creating malware which works this way, exploiting user-gullibility.
Secondly, an application can work all kinds of mayhem without ever needing admin privs. Such as, for example, reading every address in Address Book, sending a copy of itself to all your contacts, and then deleting every file in your Documents folder. No Admin privs required. There is nothing in OS X to stop someone writing something like this.
We Mac users have been incredibly lucky, in that we've not had a serious malware problem. But if this (relatively benign) piece of malware teaches us anything, it should be to take security serious and stop shouting down anyone who says there's the potential for a problem with cries of "FUD!" Taking security seriously NOW will ensure that we don't have a bad problem in the future.
