Mac OS X quietly¹ supports adding Passbook passes to your iPhone, thanks to iCloud sync. If you see a Passbook prompt in, say, an event registration email from Tito or Eventbrite, or a site like Second Gear’s clever teaser² for a new app, you can click it to make a Passbook prompt appear. If you click Add to Passbook in that prompt, iCloud will sync the pass to your iPhone.
1. I say ‘quietly’ because there’s no mention on apple.com/osx
2. For developers: that teaser site is powered by a new Passbook service Justin Williams created, called PitPass