I have been receiving sporadic reports of this over the paste few months, and even JenSense spotted this earlier this month. Google seems to be testing, I think in the UK, yellow backgrounds for their AdWords ads at the top of the results. Typically these top AdWords ads are blue.
Standard Blue AdWords Ad:
New Yellow AdWords Ad:
Both colors work for me, I just hope they don't rotate the colors on reloads for searchers.
Forum discussion at WebmasterWorld.

Comments:
gabs
03/22/2007 01:09 pm
I was sure this has been around for a long time.. More than a few month imho..
Barry Schwartz
03/22/2007 02:44 pm
I would not doubt that.
No Name
03/22/2007 03:41 pm
I have seen it as well. I wrote about it at: http://www.ittechnology.us/google-adwords-testy-czy-blad/. I noticed that I got blue background when I used Internet Explorer and I got yellow one for the same search when I used Firefox.
Dez Blanchfield
03/22/2007 09:48 pm
I hate to tell you this but I've been using this color combination for years, litterally years, and it's been a very successful color combination, and users have reported regularly that they find the light yellow color easy on the eyes and a nice clean way to differenciate between sponsored paid listings ( aka ads ) and standard SERP results. Check it out on my Australian search engine at: http://WebSearch.COM.AU WebSearch AU - The Australian Search Engine Would love to hear your thoughts on how we're doing by the way! Cheers, Dez ps: be sure to check out the "About Us" pages and the "Dez" page for a laugh at yours truely ;-)
No Name
03/22/2007 11:35 pm
In my opinion it is too easy to to differenciate between sponsored paid listings and standard results. In first months the CTR of those ads could be hight, but then it is going to be very low.
joe
04/08/2007 05:24 pm
it is very low for us since this yellow appeared