I am going to be pedantic for a moment.
The App name in Mountain Lion is Messages.
iMessages is the service run by Apple that can iMessage between the Messages apps on iOS and Mac OS X
iMessages is just another service that the Messages App can join to send either iMessages or IM (Instant Messages) to AIM, Jabber or Yahoo Buddies.
You can make it display the Buddy lists for AIM, Yahoo, Jabber and Bonjour either as a combined list (default) or separately.
As you can see lower left of the Messages Main window it lists 10 accounts
I have managed to arrange 9 of these in this Screen Shot.
The Title of each Buddy List window is controlled in Messages Menu > Preferences > Accounts in the Account Info tab and the Description text field.
The iMessage part uses/registers an Apple ID can can only text Chat to other iMessages "Accounts" which may be registered iPhone Numbers or other registered Apple IDs.
These "Accounts" are accessed by your Contacts app list (if you have the ID or iPhone number and they registered) and not any Buddy list.
The Buddy lists Buddies can be Videoed within Messages like you can in iChat) or Audio Chat.
If you are iMessaging someone and click the Video iCon then it starts up FaceTime for Video only chats on a 1-1 basis only (unlike the "ichat Style" which can do up to 4).
Screen Sharing and iChat Theatre is still possible (although the Menu item for iChat Theatre has gone).
And AIM account will still send SMS messages to a Telephone (Cell/Mobile) in your Buddy list if the person is on the "right" Carrier in the United States (has not changed from the iChat limits to this).
Messages even continues the Version numbers.
Lion had iChat 6
Mountain Lion has Messages listing version 7 (or 7.0.1 at time of writing in OS X 10.8.2)
Basically iMessage has be shoehorned into iChat and the name changed.
To make iMessage more "Up front" it presents registering an iMessage name first on First Run.
It also changes the function of the Chat window making it and Interface window as well.
As I said Buddy Lists are not in view by default.
All Chats remain service centered (AIM to AIM, Jabber to Jabber or iMessage to iMessage)
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