Nexttut shows an easy way to create that pixel art style with Arnold.
This new tutorial from NextTut shows a straightforward but handy trick to get that coveted pixel art style in an Arnold render. The technique uses the Arnold Toon Shader and a negative camera AA setting in the render settings to get that blocky, pixelated aesthetic.
A little while back, Arnold rendering specialist Lee Griggs showed something similar and drops a negative value in the Camera AA setting to get that highly sought-after pixel art look.