Two foreign keys, both to the same field in the same table, i take it this is possible to do with your version of the datamapper. So when the user is in sent items i can go $user->message->from and if i were in inbox i would do $user->message->to or however you do it, but something along those lines.
The principle applies to a number of other tables within my databse, so i would really appreciate your 2 pence on this one.