What to do if I sold my MacBook Air and didn't disable iCloud I recently sold my MacBook Air, and I deauthorized iTunes, wiped the hard drive, and reinstalled snow leopard (the os version that came installed with the MBA). 

I stumbled upon this shortly after, which made me very nervous since I didn't do it. 

http://support.apple.com/kb/HT5189

This is what I didn't do:


  Turn off Find My Mac and sign out of iCloud
  
  To turn off the iCloud service on your Mac:
  
  Choose System Preferences > iCloud. Deselect Find My Mac to disconnect
  your devices from iCloud. Click the Sign Out button on the left side.
  The system automatically removes iCloud data from all of your devices.


How nervous about this should I be? Also, is there anything I can do now that the computer is not in my possession?
Not at all - when you wiped the drive, the iCloud registration token for that device was wiped as well.

The only remnants of your account on that Mac might be your FindMyMac token that gets stored on the Recovery HD.

You might contact the seller and lest them choose whether you will send a remote wipe at their convenience to clear that. If they enable Find My Mac - they will see your email and be able to remove your hold on tracking at device. 

If you can't or don't want to contact the buyer, you could also just forget that device using the web interface to iCloud or the Find My Phone app on iOS. 

Again, the person can't get at your information or log in as you - just that computer might still check in depending on how exactly you wiped the storage drive.