I've done a piece on X inactivation -- the process which gives the tortoiseshell cat its black / orange patterns -- for Chemistry World, which you can now read here:

http://www.rsc.org/chemistryworld/News/2007/February/23020701.asp

Surprisingly it has turned out that not the inactivated but the active genes are most heavily labelled with methyl groups.

Plus, I love the idea of the tortoiseshell cat as a huge microarray for X inactivation :)