It depends on what extactly you mean by Log Out.
Quitting iChat will Log Out all accounts that you have logged in.
Clicking where it says Available (Or Away and Selecting Off Line from that menu will log that account only Off Line.
Your AIM Accounts (Those that Login to AIM) can also be set to Invisible. You can see your Buddies ON Line but they cannot see you.
iChat will Launch if you have the Computer ON and iChat unopened when someone sends you an IM.
This is because a support app called iChatAgent (Snow Leopard and Earlier) or IMAgent in Lion effectively "Listens" for such an event.
On you First Off Line IM from an AIM Buddy tyou will get an Accompanying IM from AIMSysMessage asking if you want to receive more.
Answering "No" will stop Off Line IMs (Those that come when you open the computer).
You can only turn this On again at AIM Settings (You can use it to turn them Off if you are not sure - you need to Login there).
IN addition there is a setting in the iChat Menu > Preferences > General Setting that states about setting your Status to Off Line when you Quit iChat.
This tends to stop those IMs that launch iChat when you have the computer On.
You can Click and hold the iChat icon in the DOCK and select to set an Account Off Line there.
You can also do this in the iChat Menu > Accounts
If in the iChat Menu > Preferences > General Section you set it to Display the Menu Bar Status an Icon appears on the clock side of the Menu Bar and this is a drop Down menu as well.
If a Buddy List is the Front Window you can set Off line there as well.
You can go to iChat Menu > Preferences > Accounts and untick (Deselect) the box that says "Use this Account" for any type of Buddy List and it will be logged out.
Basically you have to use a Third Party app and do a Service Discovery and find out if the Jabber server you are using supports Off Line IMs
If there is an option to Register for that you would be able to set that in the Third Party app to turn it Off or On.
I may have forgotten a method I don't use.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb( 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously