So odd.
In 10.8, any iMessage sent from an iOS 6 device is sent to OS X 10.8, as long as email addresses are associated with your cell number (this appears when setting up Messages/FaceTime in iOS). Everything is now synced, so messages sent from your iPad and/or iPhone 4S will be immediately on your Mac, and if the contact isn't in your Contacts app, it will show as a number only (you can add that number to your contacts manually).
It's confusing lol. A friend who just switched to a 4S for work from Android is getting used to it. We texted, him on his Google Talk account and me from Messages in OS X. One issue seems if you have a contact with gmail/iMessages-iCloud/AIM address/accounts, you can't determine what account you're communicating with in a conversation in Messages. You can configure all your devices and Messages app to start a conversation with either your phone number or one of your associated email addresses (I have my number and @me.com and @icloud.com all linked). Yet if someone leaves a conversation, ex logs out of Google then logs into iCloud, it doesn't indicate that in Messages.
Aside from that, it seems many are suddenly getting visual voicemail messages they weren't from the servers as well as many messages. Don't know what the issue is but may be server related as well.