Revenue earned vs. number of ad spots on a site is probably an inverted parabola; after a certain point is reached, each successive ad spot actually reduces revenue along with pageviews.

As a side note, here are several good case studies for adsense users: https://www.google.com/adsense/success

Not as annoying as the ACM charging people to read articles. I bet that doesn't last too much longer.

It's quite ridiculous. A bit sleazy, even.

( a popular read for the unfamiliar: http://www.nhplace.com/kent/PFAQ/acm.html )