Betty:

A Corian or other solid surface shower with coved corners will appear virtually seamless. No problem getting it installed either. Not cheap, but the best shower walls possible, bar none.

I'd like to add my recommendation for a Corian shower. Mine was installed a year ago. Three Corian walls (antarctica color) with caulk at the corners. I also have a custom, matching Corian shower pan. I love how easy it is to maintain -- no dealing with grout and mold anymore. It always looks and feels clean with minimal effort.