For whatever it may be worth to you, I use the InCase rubbery case that covers the back, sides, and metal bezel edges of the front of my 1st Gen iPod Touch, leaving the glass portion completely exposed. I've been reasonably careful with it, but I don't take any special precautions to protect the glass, and it still looks perfect. I think it's pretty difficult to scratch.
Ironically, the back of the iPod is what shows signs of wear, despite being in a case essentially continuously, literally from the first minute I took it out of the box. What happens is any grit or other debris that gets between the iPod and the form-fitting case will cause abrasions due to the slight motion that can happen between the iPod and the case. I think the only way to prevent that is to use a self-adhesive type of protector.
Since I keep mine in the case all the time anyway, I'm thinking of putting some of that blue painter's tape (that has a temporary type adhesive, like a post-it note) on the stainless steel parts to protect them from any further abrasion from the 'protective' case. This would be cheap and replaceable protection that would be hidden by the case. I just wish I'd thought to do it when I first got the iPod!
