Over the last several months, I've seen folks post SQL hacks to modify TFS databases, usually to try to work around some limitation such as not being able to delete work item types.  Marcel de Vries, a Microsoft MVP who has been using TFS for quite a while, recently wrote a post on his experience in trying to do this.  As he mentions, there are many relationships and dependencies that are not obvious, and sometimes you won't find the problems until much later.

Don’t change a TFS database by hand; How I got myself in real trouble