Simon engraved the back of his macbook air using the Engraving setting.
Speed was 200, Power was 80.
The effect looks good in bright light (when it looks black) The 'whiteness' of the etching is inconsistent, looking dirty/smokey in areas where it was denser. I'm guessing maybe some of the smoke? got annodized into the metal? Anyway it was on the back and just a test.
A couple of pictures are up at 
Planning on what to do to the front. 
Patrick engraved the front of a MacBook-pro
Speed was sloooow, power was 100, and the design was barely visible after over an hour of lasing time.
Maybe the anodised aluminium has changed in recent years?