Now, we have seen Google move from the old blue background to the yellow one in 2007. We have also seen Google test green backgrounds and then even test going back to a flavor of blue and yellow backgrounds together.
But now I am seeing reports of a gray background behind the AdWords ads. I have not seen it myself, nor have a seen a screen shot, but I do hope to post one as soon as someone sends one to me.
There are even additional reports in the WebmasterWorld as seeing a "light pink background" but that might be similar to seeing blue by someone with color blindness?
In any event, have you seen the gray background?
Forum discussion at WebmasterWorld.
Update: Here is a pink background picture from @marcoziero: