I disagree. I agree with your central premise (the goal these days is to get malware on the box), but disagree that Windows provides a lower barrier of entry.
Well I meant a lower barrier in the sense that anyone who was writing malware for XP will already have skills in Win32 and an existing code base that can be reused or modified. So even if we assume that Vista and Mac OS X had comparable market share, it doesn't
follow that they are in the same position as far as being a potential target goes.