Only the iMessages app in Mountain Lion on a Mac can send iMessages to iPhone and iPads (and some iPod Touches).
An AIM account login in iChat could Text chat to a Web Browser login (AIM express) or to a dedicated AIM app for that particular Mobile Device.
An iPhone user may also be able to do the same with a Google Login or a dedicated Jabber app.
FaceTime was an Add-on App that could be downloaded for Show Leopard and is included in Lion (And Mountain Lion).
You would be able to Video chat from your Mac to iPhones and iPads this way.
You would have to "register" an Apple ID to "Call" other Apple IDs or iPhone numbers that your Contacts had "registered"
For iMessages in Messages and FaceTime Apple uses a direct link with the Address Book (Contacts app in Mountain Lion) to use as a call list. (rather than the Buddy Lists iChat uses).
8:19 PM Sunday; May 26, 2013
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3) G4/1GhzDual MDD (Leopard 10.5.8) MacBookPro 2Gb (Snow Leopard 10.6.8) Mac OS X (10.6.8), Couple of iPhones and an iPad "Limit the Logs to the Bits above Binary Images." No, Seriously
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