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 test@example.com 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.
Best Answer
Unfortunately, Messages for Mac only sends iMessages, and unlike Google Voice, doesn't send SMS messages. This limits Mac Messages to only be able to send and receive iMessages from other iMessage enabled devices (such as iPhones, iPods, iPads, and Macs). Also, another result of this is the fact that only iMessage threads will sync over to your Mac - SMS threads won't (as you have noticed).
This is indeed a limitation of the software, and not a configuration problem.