Well, they say there's more than one way to skin a cat.
But I personally find it easiest (and least messy) to do it with a zipper.
In this tutorial, I'm gonna show how I pull off the unzipping effect using quick, simple methods, to give my kitty here a fresh new start on life.

1. So let's get right into this thing and open her up.
I started off by selecting the top half of the kitty's face and pasting it onto a new layer.
Then, using the Distort tool, I offset the face in an angle as shown here.

Then did the same thing to the bottom half so that both halves form a fan, framing the original face.

Using the Eraser tool set at 100% opacity, I spent a little time cleaning the edges of the kitty and also extracted the eyes from the sockets of our newly pasted face halves.

2. At this point, I decided to change the color of the inner cat to help differentiate between the two
Let's go with a cute new grey kitten. I copied the inner cat's face onto a new layer. Then, using the Lasso tool I selected the areas I wanted to remain saturated (ie. the eyes, nose).
I then inverted the selection (Ctrl + i) and with our kitty's face now in selection, I dropped the saturation level of this layer by -80.

3. Next was to add some basic shadows on the cat's face. I did this simply by taking a soft black brush (set to about 15% opacity) and painting over the edges and crevaces on a new layer.
I set this layer's blend mode to "overlay" to give the inner kitty a little more pop.

Then, with the same brush (this time set much higher in opacity, 40-50%), I went on to darken the shadows around the edges.

4. Now, it's finally time to add the zipper in. This step is probably the easiest part, in fact.
I imported a source pic of a zipper that generally fit the perspective I was looking for.
I extracted the zipper's pull tab first and copied it onto a new layer. Using the Move tool, I rotated and resized it (note: holding shift to constraint the zipper's proportions) and placed it so it would situate under our human's finger.

I did the same thing for the actual teeth of the zipper, taking small segments at a time and placing it around the edge of our outer cat's face.
Emphasis on the "small segments". Especially when following along the more curvy paths and using limited zipper sources, though it may seem tedious, keeping your segments small and numerous will leave you with complete control over these trouble areas.

I did this until both sides were completely lined with zipper teeth.

And lastly, I copied the tip of the finger off our original source and pasted it over the zipper's pull tab to situate the tab underneath.

And there you have it. Thanks to your friends at Aviary, a quick, easy, efficient way to skin a cat.

Sources:

Kitty from http://www.sxc.hu
Zipper from istockphoto.com
http://www.istockphoto.com/file_closeup.php?id=1371221