How can I convert .ichat logs to text files?
The files that Messages saves have an ichat file extension.

But from the command line you can inspect them to see they are actually binary plist files:

$ file transcript.ichat
transcript.ichat: Apple binary property list


Turns out, you can easily convert binary plist files into human-readable XML documents:

$ plutil -convert xml1 transcript.ichat


Now, when you run file again, you can see it's an XML file:

$ file transcript.ichat
transcript.ichat: XML document text


Go ahead—open it and see for yourself:

$ open transcript.ichat