Getting the AI to declare war more often is a simple (but tedious) change to the leaders in the Civ4LeaderHeadInfos.xml file. It's a tough call to make behavior like that default since a lot of players like the game as it is. My personal issue with the game wasn't that they never attacked me but that they rarely went to war with each other either.
At any rate, I plan on working on a more aggressive AI for my own mod, I'll try to work up a 'vanila' version that will work with BTS games not running my mod.