Introduction
From the moment I saw the awesome video of Gnarls Barkley, Crazy, I just knew I had to make a tutorial about it. And judging by the hits, many people agree with me.
You can view the music-video here for verification and perhaps also inspiration.

Thresholding
Assuming that you're willing to modify your own head into the inkblot, or someone elses head, we're gonna need a photo. Perferably a photo that's twice the size of your final document.
That goes for all the images you will be using by the way.

What you need to do is open your photo and go to Image>Adjustments>Threshold and notice that when you move the slider, you will see more or less of your thresholded photo. Each photo is different, so I cannot give an exact number. Just fiddle around with the slider untill you are pleased.

To get the face into your document, go to Select>Color Range and click anywhere on the black area. This will select ALL the blackness. That way when you press OK, you can just press CTRL+C and CTRL+V in your new document and it's copy-pasted!
Then just use your Eraser to remove everything you don't need like background-ornaments.

Creating your inkblot
Next up is trying to find some photo's that will function as ink-shapes. I used a random abstract wallpaper I found in my wallpaper-directory, but you can use anything from people to environments to plants. As long as the shapes are somewhat organic and playfull. Again, use the same technique as we used on the photo in the beginning and place them in your document.
Make sure that apart from the head you make everything symetrical. That means that we will need to duplicate every shape and flip it. We do that using CTRL+J to duplicate the layer selected and CTRL+T to transform it. Once in transformation-mode, rightclick and choose "Flip Horizontally".

Decorate your document with actual splatter using the standard splatter-shape in Photoshop or maybe even some custom brushes you have.

Making it look like ink
Merge all the layers, except the background, together using CTRL+E. Then press CTRL+J to duplicate the layer and go to Filter>Blur>Gaussian Blur and choose a setting of about 2-3.
Go to Filter>Grain and set the intensity to 100, the contrast to 0 and the type to "horizontal".

Then go to Filter>Blur>Motion Blur and set the angle to 90 degrees and a very low setting. Then apply the same effect, only now with the setting set to 0.
Place this layer behind all the other layers, but above the background.

Finalising it
Apply the following Gradient-Overlay onto the top layer. For this example I've used a color-combination of Green, Blue and Red. But if you take a close look at the video, you can see more color-examples like Yellow-Purple or Green-Blue.



Conclusion
And that's your Gnarls Barkley inkblot for you :)
As I said, you can try and experiment with different colors as well, beside this combination. For the real diehards, it might be an awesome idea to make an animated version. I'm not gonna touch that with a ten-foot-pole, since that requires AWESOME animation-skills.
Post your version in the comments and all the cool ones, I will feature in this tutorial. Thanks for reading and have fun!