Note: this entry is a writing of my own speculations. Stating that now so that I don't get any rumors started
XNA is the name given to a platform that Microsoft has create for making games. Versions of XNA run on the Xbox 360, Windows PCs, and the Zunes (yes, *all* the Zunes including the original Zune30). Unfortunatly Windows Phones were left XNA-less.
There's no official word on why it wasn't available for Windows phones. One can only speculate. There were speculations that Microsoft saw Windows Phone as less important. I once thought about trying to port XNA myself. But when I considered the various levels of hardware acceleration available to Windows Phones (from no acceleration with very little video memory to higher resolution OpenGL 2.0 video hardware) and the various computational capabilities of the processors within them it looked like it would be a challenge to implement a version usable on all Windows phones. Even an operation as simple as rotating a bitmap can be computationally intensive, especially floating point unit.
With todays Windows Phone 7 series announcement there was nothing said about XNA. But the the Windows 7 Phone unit demonstrated today was full of the essence of Zune. It had me wondering of an updated XNA for Zune would work on this device. That's when I had the thought Perhaps instead of bringing XNA to Windows Phone Microsoft is bringing Windows Phone to XNA! There's no telling. We will all have to wait for Microsoft's next big developer event to know for sure.
Tags: Windows Mobile, Windows Phone