The Zune mystery thickens. Our CrunchGear rumorists have discovered that when you share a song via Wi-Fi using Zune’s three day/three play system AND the other party purchases the song later in the iZunes Music Store (IZMS), you get a credit that you can later trade in for music and media. Very clever, Microsoft, very clever.

Clearly the goal here is to create a bit of viral marketing for music and, as an added bonus, drive sales on the IZMS. As we look into the Zune more closely, it seems the MS team might have just hit on the iPod killing factors that most MP3 players have been missing thus far, although we’re still fairly excited about the touchscreen iPod rumored for this year. Perhaps a Zune/TouchPod ThunderDome is in order, with the Zune flinging DRMed pig waste at the iPod while Steve Jobs and Bill Gates act as Master and Blaster, respectively. Melinda can be Aunt Entity.