Just for fun, here's another nail in the "OS X has perfect security" coffin;
http://www.theregister.co.uk/2007/09/12/quicktime_vulnerability_attacks_firefox/
A security flaw that exposes macs, running apple software, to a driveby remote code execution! Of course this is unlikely to ever get exploited, but only because of economics. If you're a malware writer, getting paid per machine you compromise, it makes a lot more sense to target 90% of the market rather than 3%.
If OS X had the same market share as windows, people would start exploiting these vulnerabilities. More pertinently, they'd start writing trojans which can't be meaningfully defended against with permissions or OS design, without making it inconvenient to install legitimate software. Trojans, be they distributed by websites, p2p or mail worms, are the biggest risk to windows machines these days - drive by downloads and slammer style worms are a non issue to fully patched systems.