Looks like Google is once again tinkering with their AdWords background colors. Have a look at the new background color:
Google introduced the pale yellow AdWords background in 2007, which was replaced last July by a light purple background:
It's not yet clear if this is a permanent change or one of Google's tests to see if it results in higher click-through rates. Also seems like this could be another change meant to blur the lines between paid and organic results.