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iChat only saves the conversations if you have the "logging" feature turned on which is in your Preferences for iChat.
 
yes. There are two ways to view this. One, is go to documents, then iChats if its already enabled. or you can go ichat, preferences, messages, and at the bottom, if the last box is checked (on tiger) click show folder to the left. If not, check it and it will start recording. Those transcipts have come in usefull for me in the past. And spotlight made it alot easier for me to check them.
 
Yes, spotlight and smart folders have really made the logging option useful. It used to be a daunting task to try and go look for something said a couple days ago.
 
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