I just recently purchased a Swiss sapphire caseback for my GMT-C and installed it over the weekend. The caseback on my GMT-C is pretty cool. I added the installation steps to my DIY page at http://www.minus4plus6.com/caseback.htm#116710 along with a comparison of the caseback made in Germany.
I know some feel that clear casebacks are sacrilege, but in my mind there is no other way to wonder at the micromechanics inside.
All spelling/photo errors are mine, so feel free to point them out.
(BTW, I also added some cool watch item reviews over Spring Break)