Windows Mobile 5.0 Pocket PC Edition today screen customisations:
Windows Mobile 5.0 Smartphone Edition homescreen customisations:
Although the second link refers to the Smartphone 2002 version, many things seem to be quite similar.
I think what’s interesting is that you can create custom Smartphone homescreens using just a single background image, and an XML file. That makes things pretty easy to create.
There are apps out there that can help you create themes for either the Smartphone or the Pocket PC.
But I think for the time being, if you only want to dip your toes in the water without wanting to fork out any money, the guides from Xda-Developers and MoDaCo should get you started.
If you’d rather just download themes rather than making your own, then here’s a few sites you can download free ones: