NOTE: This is being phased-out in favor of the new topic system. I have locked changes to bump lists. Soon they will be converted to topics, and threads bumped to these lists will be added to those topics.They want to start using the "keywords" present at the top of every thread for tracking purposes.
This makes sense from a database standpoint. Each thread will have only a few keywords tied uniquely to that thread in an easily indexed field, as opposed to having to index the millions of comments on tens of thousands of thread, looking just for the ones pinged to bump_lists.