I had this issue forever and 10.8.2 solved everything so beautifully. iMessages finally works absolutely perfectly w/ my MBA and iPhone 4.
1. Go to Settings -> Messages -> Send & Receive
Sign in w/ your Apple ID.
Under where it says "You can be reached by iMessages at:" make sure your phone number and your Apple ID are both present.
2. Go to Messages on your computer and open preferences (Command ,)
Your Apple ID should be the same as on your phone.
If you've updated to the latest 10.8.2, you should also see your phone number listed along with your Apple ID under the "You can be reached for messages at:"
If it's not there try signing out of the account and signing in again. If you add an email on one device, you'll get a notification on the other saying that "_____'s MacBook pro has added
_____@___.com" or something like that.
The same goes for FaceTime on the computer! Now if you add your number+Apple ID on both phone and computer, when you get a FaceTime call it will ring on both!
So excited about it finally working as advertised. I wish iCloud tabs would work as smoothly.
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This.