I receive regular updates from Media Monitors about top radio advertisers. We don’t post the info at Radio magazine because it’s not really tech related. It’s also covered just fine in the management outlets.
Some companies purchase a great deal of radio advertising. Walmart and Geico are two that come to mind. A recent Media Monitors list ranked the biggest radio advertisers of 2008, and the names aren’t really surprising, although there is some false hope in one. Here’s the list:

It’s good to see HD Radio being promoted so heavily, but at the same time, there’s no money changing hands for these ads. Stations are committed to running the spots, but the volume of unsold inventory also raises a flag for our struggling economy.
The news is a mixed message. Good for HD Radio. Telling for radio advertising.