http://www.codeplex.com/PocketCBZ
Incidentally, it looks like
Jay Kimble seems to have gotten a CBZ viewer for Windows Mobile out. It uses .NET CF 2, however.
Edit: One thing I'll note - it's worthless for larger files, and they apparently WANT you to use the VB program that you get off the same page to convert the files to 640x480, as anything larger in scan size will cause the program to have a fit and die. It also doesn't have a fullscreen mode.