I use a (black) Speck ToughSkin 80/120 GB Case for my Zune 120 and it works great for me. It has no flaps, it comes with a rotating belt clip (makes a good stand too when I want to set it down while I listen), and the case provides plenty of protection for my Zune. Here's a link to it:
Speck | ToughSkin Case for Zune 80GB/120GB