One AdSense publisher claims seeing a Google AdSense ad with the "ad" label replacing the favicon spot in the AdSense ad format.
He posted about it in WebmasterWorld saying:
Anyone else seeing this new test on text ads. Instead of the image/favicon next to the Advertiser URL, I am seeing a black square that says "Ad" on the AdSense ad. Similar to how traditional Adwords results have the yellow "Ad" label.
No one else really saw it and I was unable to secure a real screen shot but I hacked one together based on what I think this person was talking about.
That "ad" label is in the place of the favicon taken from the advertisers site.
Again, I have not seen this myself in action but it is said to match the AdWords ad label look.
Forum discussion at WebmasterWorld.