I was some how left out of this endeavor to AMD this week, but no matter.  Anandtech has a good quality article up on what comments they got from Fred Weber, who was the lead architect on the K8 core.

We also asked Weber about his thoughts on wafer and die stacking; he sounded particularly interested in them, but added that for a microprocessor it’s far too early to count on die stacking because of yield concerns.  He said that the time for the technology to be used on microprocessors would only exist once there’s mass market use of it in memory manufacturing, then and only then would it be mature enough to migrate to microprocessors.