For a much faster solution than ImageMagick, you could use the GPUImageToonFilter from my GPUImage framework:
It combines Sobel edge detection with posterization of the image to give a good cartoon-like feel. As implemented in this framework, it's fast enough to run on realtime video from the iPhone's camera, and is probably at least an order of magnitude faster than something similar in ImageMagick. My framework's also a little easier to integrate with an iOS project than ImageMagick.
If you want more of an abstract look to the image, the GPUImageKuwaharaFilter converts images into an oil painting style, as I show in this answer.
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