I was moving (or should I say trying to move) the google ads on some of my pages yesterday, and again this morning. For adlinks, I have one 728x15 running across the top of my page. I have been able to add any other combination of ads with this for my total of 3, as long as I dont try to add a second adlink. In the case of adding an adlink code, it simply wont display, not matter what I do.
It's right on the AdSense program policies page: No need to ask Google, it's spelled out pretty clearly on that page.
As I and others have said before, horizontal link units work best, especially when they're near navigational elements at the top of a page.