I have a tutorial over at E-scapeandScrap's blog today featuring painterly photos mixed with line drawings/sketches. In a nutshell, you need a black and white line drawing of your original image and a painterly version of the original. The black and white is layered over the painterly and the blend mode set to darken or multiply. Here are six more samples of this style of work.
Remember you can always add more adjustments to tweak your image.