I'm new to modding in Civ IV, so this might be common knowledge, but does BetterAI (and mods in general) work on multiplayer? If so, I would assume everyone has to have it?

This thread (http://forums.civfanatics.com/showthread.php?t=208277) is a good starting point. :) There was a fair amount of work done on synch problems but I don't think they were totally resolved.

I've been playing multiplayer almost exclusively since the 2/12 build with no problems....probably around 20 games or so. And yep, everyone has to have the same build of Better AI installed.

The OOS errors still exist in 2/12. Though a lot may have to do with the type of games you play. For example, if you played your games turn based, you will likely encounter the OOS errors. You might not see the OOS errors if you play your turns simultaneously. I was able to pinpoint the problem in the code and Iustus was aware of what was going on, though fixing it was not as easy as one might think. He then got his new job and hasn't had a chance to get to it.