One more nail in the Microsoft Zune coffin emerges from a close reading of an interview with a Microsoft exec.

The only feature that distinguishes a Zune from an iPod is the built-in 802.11 wi-fi that enables sharing songs with another Zune owner. It was made clear that a song purchased from the accompanying online store can only be played on the friend’s Zune three times, followed by advertising on the friend’s computer.

That left an open question about sharing your own unprotected content – mp3 files with no DRM or license restrictions. If they arrived at the friend’s Zune without restrictions, that would be a powerful way to share music and other media files.

According to this report, that process “probably involves the introduction of a DRM layer.” In other words, the mp3 file will be converted to a DRM-restricted format, or the Zune will have some built-in mechanism to restrict the number of times the mp3 file can be played (and presumably prevent the mp3 file from being downloaded from the Zune later). (Confirmed in this blog post from a member of the Zune team.)

If so, Zune file-sharing is dead on arrival, another victim of DRM restrictions blocking consumers from useful, desirable technology.