Bill Wittress over at the newly-logo'd ZuneGuy,
solves the mystery:
"I've seen a few posts today about the Yellow (or Citron) Zunes. I've also seen a lot of speculation as to whether this is a "hidden" product. I'll shed a little light on this. They do exist. They are the ship-gifts given to the team for launching the product. Just like last year when we gave out Magenta and Orange devices, these are a special run just for us. There you have it, mystery solved."Damn. I really liked the way they looked!