When you return to your computer, is it in sleep mode (power light pulsating slowly), screen saver, or has the computer logged you back out to the password screen?
If it's in sleep mode, as posted above, there's no way around the computer signing you out if it enters sleep mode...sleep mode is disconnected from the world. If it's in screensaver or logoff mode, you'll need to change the preferences inside of iChat for when you want it to log you off.
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