I agree that OS/X is harder target for Malware then Windows, because OS/X is a better OS.
Ha - just open Console.app and see those double free()'s and other fun stuff flying by if you understand what I mean. Point is that there is no acceptable proof that OSX is more secure than Vista or Win7 or Linux for that matter.
You never "claim" security, you only keep doing your best to "achieve" security for it's like the battle against flu - before you declare success against a trace of flu there is a new trace born somewhere that just blows away all the existing defenses.