It's my understanding that for Feeds to support node_reference and user_reference fields in Drupal 7, a mapper will be needed. This is effectively the same issue as #724536: Mapper for nodereference field in Drupal 6, but for Drupal 7 instead of Drupal 6. I'm bundling node_reference and user_reference together since (for now at least), support for both is being provided by the same module, namely References.
I've taken a first stab at creating the necessary mapper. (It's also my first time writing code for Drupal, so apologies for any standards I've overlooked). So far I've only done a few basic tests -- and I'm guessing that a proper suite of test functions will eventually be needed. But I wanted to post what I've done before going too much further in case there's any feedback.