Why SQL Server doesn't do this automaticly escapes me. If you ever restore a database you end up with a bunch of users in your database, and a bunch of users defined on your database server (well only if they were already on the server). But they are broken half users.
Invaribly I spend half an hour trying to remember the name of the stored procedure that fixes this up when ever I restore a database.
This post is so I can find it quicker.