A few months ago, I wrote a quick and dirty script that goes through iTunes’ iPhone backup, searches for Beejive’s data and converts it to Adium-compatible chat logs. It even imports files and pictures. It does not support timezones and daylight savings time, but other than that, it does a good job. I was initially planning for writing a converter to iChat log format, but iChat’s transcripts basically consists of serialized data stored in plists and is undocumented, so it would be very difficult to implement iChat’s transcript format.
You may download it here. If you find a bug, please do let me know in the comments, but don’t expect I’ll fix it anytime soon .