SimGuruGraham has confirmed that The Sims 3 is compatible with Windows 8 when you have the latest Sims 3 Update (1.39). With Windows 8 already released, those of you who will upgrade should make sure to backup your Sims 3 Files prior to the upgrade. Better safe than sorry!
Sims 3 supports Windows 8 when played with the latest update. RT @crinrict How is the compatibility of the game with Windows 8 ?
— Graham Nardone (@SimGuruGraham) October 24, 2012