Google recently announced an upcoming change to the Adwords program, which will allow advertisers to target contextual advertising on specific web sites.
Good article from Sitepoint.com about the new Google program.
Google’s ingenious model for selecting ads to display (maximizing revenue per impression) allows them to perform the ‘apples to oranges’ comparison with ease, and allow advertisers to bid on the same inventory in whichever terms they find more comfortable.
The biggest improvement, though, is that advertisers will be able to run separate site-targeted campaigns.