Now on Google AdSense text ad units you can learn the URL of the advertiser by hovering your mouse pointer over the ad. It appears Google added a form of title or alt tag element to the ad which reveals the advertisers URL without having to click on the ad.

Here is a picture:

A WebmasterWorld thread has most Google AdSense publishers claiming this is new.

They find it useful in that they do not have to click on the ad or right click and copy the ad string URL to determine who the advertiser is. This comes in handy when publishers want to block certain advertisers from placing Google ads on their sites.

Forum discussion at WebmasterWorld.