There is a whole forum for them:

Walk-Away Spouse

You might find some insight in the Midlife Crisis forum as well; there seems to be similarity in behavior between an MLCing spouse and a WAS.

Me: 44, Wife: 39
M: 17 years T: 20 years
Bomb on 08/25/09
1/13/10: MC started
1/28/10, 2/8/10: More bombs
8/28/10: Wife moved out
No talk of D, no movement

"Every day is another chance to get it right."