i did a little bit of research for you on my way home. according to my friend, who is a developer of windows mobile devices, the incoming call screen is just an xml file... if i understand correctly.
if it is then just like homescreens i believe this could be edited too. now unortunatly i have not researched it more than making a phone call but at least now you have a starting place for a nice google search
EDIT: i looked around and searched my phone. unfortunatly i could not find any xml file for the incoming call screen. that doesnt mean one doesnt exist... though its totally possible that i mishead/understood him

ops: