According to an (anonymous) tipster in Engadget, Zune's display is possible to crack after leaving it to charge for many hours. This could happen when the Li-ion battery under the screen oversizes because of the heat. By the way, I've forgotten it charging many times for 24 hours or more.
Maybe its possible for Li-ion batteries to get oversized, but I've never felt the Zune hot while charging. And of course haven't noticed any cracks on the screen. Any way, if this is true and has happened to you, feel free to write a comment below.
Till then, watch this video from the Zune release days back in November.
Microsoft, Zune, Online Video