2008-05-08 Update:
Before I posted what I found, I had searched the web and found a usenet posting by Peter Zimmermann of www.ambit.ch asking the same question I was trying to answer. I sent him a note about what I'd found. He said they had dug through it and found the root of the problem is having contact with a blank last name. Indeed, I found that on the page in question there were contacts that had been imported with no last name. Thanks to Peter for the update and the further investigation.
-- end update --
After upgrading a customer from MS...