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.