As long as those borders are in your code, not theirs (i.e. as long as those borders are on a <div> or some other element in your page and you didn't alter the Adsense generated code in any way or add your own styles to it) that should be OK...
it should be okay as long as you didnt edit the adsense code. also make sure that none of the elements are hidden or covered up in different browsers .
This border problem mainly comes if you put ads with images unless if u don't edit the AdSense code nothing much to worry. nothing wrong with that ad unit.