Just to add confusion there are two different APIs for Pocket PC Today
screen and Smartphone Home screen. For the former there is an article with
code on MSDN* to show how to built one with a combination of managed and
native code (you can't write one solely in managed code).
You would have to use a similar technique for a Smartphone home screen
plugin - write it in native (C++) code and use one of the many forms of IPC
to update it's contents from your .NETCF application.

*
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnppcgen/html/T
odayScrn.asp

Peter