As is currently being discussed on this forum, WordPress.com trackbacks are broken. The WordPress trackbacks option hides and comes out like it’s supposed to but there is no trackback box to put trackbacks into. If you need to do trackbacks you can use haloscan’s manual trackback page. You will have to create a haloscan account to use it.
Update: As of 8:45PM CDT on 5/20/08 it’s fixed.