http://www.anandtech.com/weblog/showpost.aspx?i=603

Apple makes two models of the Mac Pro, one with two sockets and one with only a single LGA-1366 socket. The two socket model, often referred to as the 8-core Mac Pro, actually uses Nehalem CPUs without any heatspreaders. I suspect this is to enable them to run at their more aggressive turbo modes more frequently (high end Xeons can turbo up to higher frequencies than regular Xeons or the Core i7). The single socket model uses standard Xeons with heatspreaders, so there’s nothing special there.
MMM direct die action.