So I reverse-engineered the iChat .chat log format the other day for Logorrhea 1.1, Spiny's iChat log manager. It's a pretty simple NSArchiver layout, but it's sort of guesswork to figure out what gets archived into the objects, and where in the hierarchy they need to be unarchived.
It sure would be useful to have a tool that could generically unpack NSArchiver and NSKeyedArchiver formats, or at least give you some guess to how the the archive is laid out internally. Unfortunately, the format is proprietary, as near as I can tell.
Since Jan & I are so generous, Logorrhea is open source, and available over on the Logorrhea site.