Just a clarification for everyone of what I've noticed, because lots of commenters on this thread are talking about borders: If you use ad units without borders, but your ad unit has a different color than the background color of the page, you will still see rounded corners with this new feature.
So, although I acknowledge most users without borders on their ad units are blending and so will have the same background color and page color, if you're not, you can use rounded corners with or without borders.
And by the way, the non-ad part of the rounded corners is correctly transparent, thank you G engineers!
