at least that's what i thought initially when i saw it.  i tapped the glass though and it was still alive. 
but, with some research: 
"If you should notice the tarantula on it's back, don't fret, this is the normal molting position and the spider is not dead. The molting process may take several hours to complete. When entering a molt, your spider may refuse food for anywhere from a week to several months. It will often develop a black shiny cast to the skin of abdomen as it's new skin begins separating from the old one."
look's like in a few hours im gonna have myself a nice skin to throw at people n' freak 'em out 
Normal picture of the spider 
