This is interesting, if you search for [oil spill] in Google, you should see an ad for BP. The display URL on that ad is www.BP.com/OilSpillClean but if you click on it, you are taken to YouTube.com and not BP.com.
This clearly goes against Google AdWords display URL policy that reads:
Your display URL must accurately reflect the landing page URL of the website you're advertising. It should match the domain of your landing page so that users will know which site they'll be taken to when they click on your ad.
It seems like BP is getting away with this either through some sort of bug in Google's ad system or they were given a special consideration for this?
Here is a picture of the ad:
If you click on it, you go to YouTube.com, specifically to the BP channel on YouTube.com.
Forum discussion at WebmasterWorld.