Good news for Zune application developers! The team at ZuneBoards has been able to crack the Zune HD and older Zune models to allow for full scale application development. Rather than having to work within the confined environment provided by the XNA game development platform, no longer will devs be forced to work with no 3D, no Internet access, and hardware performance limitations.
They have released a toolkit called OpenZDK which gives lots of insight for both developers and end users. At the moment, only a few applications have been developed with the new kit but they hope that their guide will help in bringing next gen apps for the Zune line. You can check out the OpenZDK Wiki here.