Silicone Skin for Microsoft Zune HD Zune 32GB Zune 64GB + Screen Protector Kit (7 Color Available)
This is just very cheaply made.  The silicone is thin and cheap.  So much so that it feels like the Zune will fall out of it, thus defeating the purpose.  The band is also cheap.  It's not wide enough.  The combination of the cheap silicone and the narrow band results in my 80gb Zune just bouncing all over the place at even a light jog.    I've seen others of these for the 30gb and they are made of better materials and would hold the Zune in place much better.  I bought this one because the armband is optio