I found an entry at Poynter Online's Romenesko (Jim Romenesko) column.
Apparently Romenesko posted the content of an e-mail he received from Eric Alterman.
Alterman says that it is "common practice" in the advertising business to negotiate rates based on the volume of business. Alterman added that it would be an easy issue for reporters to investigate.
Does this information lay the matter to rest? Not at all. Are advertising agencies free to reject work from clients based on ideology? Could the New York Times offer the DNC a better rate on its ads than it offers the GOP based simply on the volume? How about the Green Party? Are they stuck automatically with higher rates than the GOP or Democrats could negotiate unless the Greens go through an ad agency with bulk purchasing power?
It's unclear at the moment whether Alterman just made the issue fishier or more aboveboard.