Lindsay Kerr of Solo2design has produced an example walk through of Ambient Occlusion rendering in Blender.
Available as a pdf from the Solo2design website it walks through the effect of various settings and is a great step towards understanding how to use Ambient Occlusion in blender.
Lindsay says this about the tutorial:
This is my first real attempt at writing a tutorial or something that resembles a tutorial. The subject is Ambient Occlusion, and its really aimed at the beginner while avoiding those big words. I havent actually got a link on my site yet because its due for a overhaul, so cgtalk has the first look."
Lets hope Lindsay does a tutorial on the excellent bird render shown above.