Ribbing aside, the OS X security model is, generally speaking, the Unix security model, which has been around a lot longer than Windows. Henry Spencer is attributed with saying "Those who do not understand UNIX are condemned to reinvent it -- badly." Perhaps OS X has done it badly, but perhaps not as badly as others.

I agree that lack of security problems is not indicative of good security, but Apple does seem to be making good decisions. As I said earlier, they've got some super smart security people working for them and letting them drive the security model.