Here is what I heard from the publisher on 2/7/07.  No explanation:

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Unfortunately, the following Publication has ceased. Thank you for being
such a valued subscriber. If there is anything further I may assist you
with please let me know.

Email Customer Service Rep-Ken
If replying please do so to post_graduate_medicine@halldata.com
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--Laura Davis
   University of Washington
   Health Sciences Resources
   ldavis@u.washington.edu