add API function to check if a user is signed up for a node

This sort of code appears several times and it could really do to be put into an API function:

      // See if the user is already signed up for this node.
      $signup = db_fetch_object(db_query("SELECT sl.*, n.title, u.name, u.mail FROM {signup_log} sl INNER JOIN {node} n ON sl.nid = n.nid INNER JOIN {users} u ON sl.uid = u.uid WHERE sl.uid = %d AND sl.nid = %d", $user->uid, $node->nid));
      if (empty($signup)) {

Should this also handle retrieving a user's signup details? I don't know the code well enough to know if this would be useful too.

"ack 'SELECT.*{signup_log}'' suggests that there aren't any places where details for a given user and node are loaded.