Panel 7 is also an excellent example of a "beat" panel. The lack of text, expression, or other stimulus helps carry across the feeling of "sinking in". Dilbert is here stunned by the results of his "experiment", and this feeling is passed over to the audience. It also helps establish a bit of distance between the previous six panels, all taking place in the same area, and the last one, which is not only set in a different location, but contains the punchline which gives the entire rest of the comic (i.e. the previous 7 panels) perspective.
In reference to Dilberts expression: in film this is known as the Kuleshov Effect. Show an audience a man with a blank face intercut with pictures of other things (food, an attractive woman, ect) and the audience will think his expression is changing in response to the image he's "looking at" (i.e, he looks hungry, desirous, ect).
I think SA did a great job using this effect in this comic.