I was recently working with a customer and we were defining some new content types. As you probably know, you always have to inherit a content type from an existing one. This is fine, but sometime you don't want the first column to be called Title. Well, we changed it, and guess what... all new lists created bellow this site, all had Title exchanged for our new name, in this case, "Customer Name", not very nice, for instance in a normal Task list.
Well, not a big problem, we thought, we'll just change it back. Well... we couldn't... so we were stuck with a "non-Title-name". Bad, really bad. The only way I found of resetting this, was to delete the entire sitecollection (I had changed the site column at the top site) and the creating a new one. Not a very nice workaround...
So, be ware, don't change the "Title" site column located in the group "_hidden".
CRM and SharePoint Consultant