Apple vulnerabilities will eclipse the number of Vista vulnerabilities.

I found this somewhat-recent quote by Thomas Ptacek particularly memorable:

"Apple has announced some of these changes already. But, by this time in 2008, the number of OS-native, default-install Apple vulnerabilities will eclipse the number of Vista vulnerabilities. Despite whatever hardening features Darwin and XCode add, there will be more exploitable Apple vulnerabilities announced in 2007 than will be announced for Windows XP SP2."

From Predictions 2007: Ptacek vs. Lawson