From ClicZune :

[...] Matt Jubelirer, the product manager who was responsible for the Zune player, left the Zune team last August to join Slacker as the Director of Product Marketing.

The circumstances and the reasons why Matt left the Zune team are obscured, but the timing could certainly not be worst since the market is expecting new and improved Zune players for the upcoming holiday season. His bio was also removed from Zune.net. [...]

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EDIT: After contacting Slacker, they confirmed this story. Matt Jubelirer has left Microsoft and joined Slacker.com
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I suppose we should have learned it from more official sources, as we learned about Bryan Lee. But now we know: Check the biographies page on Zune.net and if you see someone missing...

Robert Scoble had got an interview with Matt Jubelirer on October 2006, before the Zune release.

And here is another, probably not his best, interview of Jubelirer from the same period:

And another interview/presentation from on10.net :

Related articles:
Zune Product manager interview higlights (Video: Matt Jubelirer on Robert Scoble Interview )
Zune's Bryan Lee off Microsoft