Using Site A to authenticate users of Site B? Is there a module that will pass the authentication request off to a second Drupal site, ideally to an out-of-the-box services endpoint?

There are some similar questions about login integration with other systems, but I couldn't find a Drupal-to-Drupal question.
Have you checked out the Multisite Login module? This allows sharing logins across different domains, and I've had luck with it. They also have a Drupal 7 version available in dev.