How does a wifi antenna work in full-metal laptops?
They put them where ever they can find an opening and try to do a good job tuning them.  Form comes first at apple ;)   Take a look at this, they've mounted one antenna just inside the opening for the optical drive on a recent macbook pro.


Also they have a patent for a logo antenna 

I once did a tear down and comparison of a bunch of apple products antennas and they performed really well compared to others in the field.  Although there was that little problem with the iphone ;)  So I think form comes first and then it's up to the EE team to be creative and figure out how they can get it to work.

On other cheaper laptops I think you'll find they just hide them behind plastic inserts.