A thread started yesterday at Search Engine Watch forums discusses how Google's AdSense is Looking Into Negative Ad Blocking By Keywords. The thread was started by msgraph and he states:
It seems that Google is researching into allowing Adsense publishers to block ads by keywords, categories, and/or concepts. Currently you can only block ads by specific URL. If they go through with this it could be a great improvement on narrowing down the specifics for targeting. Also you would not have to research your competition as much. Only problem I can see is that some companies who do not compete with you yet have those keywords listed in their inventory could be mistakenly dropped.
In the thread he pulls out a few interesting quotes from a patent filed by Brian Axe and Narayanan Shivakumar, Google employees, named Identifying and/or blocking ads such as document-specific competitive ads.