node permission based on a cck user reference field
Install the User Reference Access module - it directly addresses this requirement:


  This module works with CCK userreference fields and the node_access
  module to add additional grants to nodes.
  
  To enable userref_access on a node, edit the userreference field for
  the node type and check the "Use User Reference Access" box.
  
  Any users that are referenced by this field will be given view
  permissions for the node, all other useres will be denied access to
  the node.