I have a new Retina Mac running Mountain Lion (10.8.2) and an iPhone running 6.0.1.
On my iPhone I can send and receive text messages from my email firstname.lastname@example.org and +1 (555) 555-5555. On my phone it has iMessage turned on, Send Read Receipts is on, Send as SMS is on, Send & Receive (2 Addresses which are my email and number), new texts sent from my number.
On my Mac iMessage I'm logged into my Apple ID and it says I can be reached from my email address or number (which are same as on my iPhone).
If an iPhone user texts me, it shows on my Phone and Mac and I can respond on my Mac or Phone.
My issue is that I cannot send or receive texts on my Mac to/from non iPhone users as I've been able to with Google Voice. I had thought with Mountain Lion you would not be able to at least receive texts sent to your phone number. Or is it still limited to only iPhone users? I would think if a non iPhone user sends to my iPhone number it would sync that message to my computer.