Hi,
Historically iChat could send SMS messages via an AIM Account.
Messages can still do this even though the Menu item no longer shows.
There are some caveats.
1) The computer has to be set as If it is in the United States (System Preferences > Text and Language > Region)
2) The Phone HAS to be in the United States
3) It has to be on a carrier that supports AIM SMS Forwarding.
You add the phone number as if it were an AIM Buddy with a leading + 1 (As in +1234567890)
I have also seen vague-ish info on whether the iMessages side will SMS to other phones.
The iMessages side can send iMessages though the Phone carrier either To or From the Phone when it is not on a WiFi network.
Generally the computer will be on a Network and is using the iPhone Number as an Alias to Send or Receive sync items.
There is no evidence it is using the Service Provider for the iPhone to send the messages at all.
There is nothing in Messages help about SMS

10:46 PM Sunday; October 7, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
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
Message was edited by: Ralph Johns (UK)