There's an interesting story over at MobileCrunch. They say that according to rumors the new Windows Mobile 7 will actually be a copy of the Zune HD kernel. It will only work on one screen size, and will "work best on OLED devices". Hardware makers will have to comply to the H/W specifications. This might mean a very big push for OLEDs if Microsoft says that it's the preferred display technology. There were rumors that the LG Apollo phone will use WinMo 7 and have a WXGA (1280x720) resolution. The Zune HD, meanwhile, has a 480x272 display.
The new OS will not be backward compatible - which means that all applications will need to be rewritten. The user interface looks just like the Zune HD's, it won't have background processing, no threading and no access to system resources for applications.