I bought several cases for my iPhone when I got it (on June 29th) and one of my favorites was the Incase Protective Cover which is like a neoprene glove for my iPhone.
Unfortunately, soon after getting the case, it started stretching and now my iPhone will barely fit inside. I called up Incase and it turns out they had a bad batch of rubber.
If you have a back protective cover that’s stretched too much to carry your iPhone snugly, fill out a warranty form and they’ll send you a new one.
P.S. In the meantime I’ve been using a Gelz Case I picked up from RadTech. It’s a fraction of the price and I like the design. Unlike the Incase (which is thick), the Gelz is very soft, thin neoprene. I just have the case to prevent scratches and normal wear, so it works great for me.