We all know that lurking inside the Zune HD is NVIDIA's potent first-gen Tegra chipset, but what isn't so well publicised is that Microsoft's XNA development platform for the PMP doesn't allow access to the hardware's full capabilities. In fact, it means no 3D, no internet access and performance apparently an order of magnitude less than what the Zune HD is capable of. That wasn't good enough for the ZuneBoards team, and so they've come up with OpenZDK, an alternative development platform to XNA.
With OpenZDK, developers can create Zune HD apps - as well as apps for the original Zune models - that take advantage of the Tegra APX2600 chipset, including using custom vertex and fragment shaders with OpenGL ES 2.0 and programming in C++. The resulting apps run on top of the regular Zune firmware, and shouldn't impact your Microsoft warranty at all.
The end result is the ability to make games that match up to Microsoft's own titles, such as PGR: Ferrari Edition and Audiosurf Tilt, rather than the more mundane possibilities that the dumbed-down XNA allows. More information on getting started at the OpenZDK wiki, and the ZuneBoards guys have a special forum (currently empty) for apps created with the new framework.