I highly doubt Sony would license WM2003SE/PPC from Microsoft. Microsoft decided to take on Sony's dominance in the game console market, aiming the XBox squarely at the PS2.
The console wars are about to get even more heated with MS's "neXtBox" battling the PS2/PS3 for dominance of the home "digital hub". These new consoles will be convergence devices delivering gaming, movies, etc; to put it simply, paid content.
This battle will also be over formats and standards; Sony with the Blu-Ray DVD & UMD and Microsoft with WindowsMediaPlayer.
Sony is coming out with the PSP, the portable counterpart to the PS2/PS3 digital hub. Why in the world would they market a portable device that would help support Microsoft's formats?
IMO Sony killed the Clie line because they couldn't fully control the OS and therefore the content (games, music, movie, etc), allowing non Sony and non DRM content to be played through other Palm OS applications on the Clie. Sony is pushing paid content in a big way with the Connect Music Store and they have plans to deliver movies and TV shows through a download service.
The PSP will fit with this paid content model for Sony with strict DRM for all of Sony's content.The Clie didn't fit well in this new paid content strategy and a PPC powered Clie most certainly wouldn't.
Edit:
Some good background info on Sony's paid content plans @
http://www.paidcontent.org/pc/arch/cat_sony.shtml