Yesterday I realized that non of my custom lists enforce content approval by default. That really worried me as 90% of the data we display in the web site which I'm working on are coming from custom lists.
Enabling this manually is a piece of cake but I wanted to include this in the list definition.
After some research and testing I found out that there are 2 places you have to configure.
You have to set ModeratedList="TRUE" for List element and EnableModeration="True" for ListTemplate element in your schema files.
Remember this works only for new lists you are creating from these definitions but wouldn't affect lists which already exist.