The overlapping would still happen even if you moved it to the top of the screen. Well, if you were in a 25+ person raid that is.
It's a known problem bought about by changes made by blizz in 4.0. Scott is working on resolving it in nUI6 but nUI5 users will need to use the aura button plugin found in the Unit Frames section of the nUI download page.
One of the several posts around the place is here :
http://www.wowinterface.com/forums/s...ad.php?t=38017