Step one:
Add a layer on top of the image on which you the paint smear to appear. Make a selection that is the approximate size of the smear you want to create. Don't worry if it's not exact, you can adjust it in a minute.

Step two:
Fill the selection with a gradient from the color you want the paint smear to be (foreground) to white (background). If you make the gradient slightly off center (only a few degrees) the paint smear will be thicker at one end than the other.

Step three:
Add noise to the gradient. The more noise you add, the more striations you'll see in the paint. (Note that this method for adding striations is straight from the brushed metal tutorials in the Photoshop User magazine and this website.)

Step four:
De-select the gradient (CTRL-D) and add a horizontal motion blur to the gradient. This give the smear that fade-in fade-out look.

Step five:
Open the Layer Options dialog, by double-clicking on the layer with the paint smear, and adjust the blend settings for This Layer. By moving the This Layer sliders to the right, you begin to eliminate the white. Separate the two sliders by holding down the [ALT] key.

Step six:
Rotate the layer 90 degrees Clockwise with Edit-Transform-90 CW. We need to shear the layer, but shear only works on a vertical plane.

Step seven:
Use the Distort->Shear filter to give the smear that artistic curve.

Step eight:
Rotate the layer 90 degrees Counter Clockwise with Edit-Transform-90 CCW.

Step nine:
Use Edit-Free Transform to size, stretch, etc. as necessary.

Step ten:
Throw some text over the top of it, and it's a great way to separate foreground text from the background.