Getting a case would definitely help keep the "smudges" off the unit But I would not worry too much about them, they won't keep the unit from working or anything.
If you want to keep it looking nice I use Endust for Electronics. Just spray some of the stuff on a soft cloth and wipe the unit off.
As for screen protectors the only ones to get are WriteShields from PocketPC Techs. Go
HERE to check them out and order them. They are kinda expensive but considering it is the middle of October and I have had one on since the middle of January with no scratches or other problems I think they are worth the price.
In additon PocketPC Tech also sells "Klear Screen" cleaner. This is some great stuff as well and can be ordered in a "kit" along with the WriteShields. I use this stuff as well for cleaning the outside of my PPC.
Good luck in keeping it "clean"