The Depths Of Penny Arcade

When did Penny Arcade start using depth of field? Today's strip really seemed to point that out for some reason:

Paint The Line, Part Four

Other examples:

Cloaked in Twilight.
Uncompromising Vision.

Are there other comics out there using dof effects? Interesting to see dof in a static image artform to focus attention when the artist has so many other techniques to do the same thing. Compare that to cinema, where dof is, if not a necessity, at least a fantastic technique to focus the viewer's attention in a moving image.

In a three panel comic that is well known for wit, humor, and vulgarity, do these dof effects help whatsoever? Would this work better in a narrative comic?