Heat sinks are now bowed under most occasions as it offers a more superior mount under pressure directly under the DIE.
Some will come flat with that mirror like finish, but bowed heat sinks and bowed blocks are vastly superior.
This was discovered a while back in the watercooling world when people would artifically induce a bow by putting a o-ring inside the inlet of the block.
Swiftech goes into it in detail here, as the first block to induce this artifical bow was the D-Tek Fusion, and Apogee.
http://www.swiftech.com/apogeegt-bow-tweaking.aspx
In CPU's where the geometrical center of the Integrated Heat Spreader (IHS) coincides with the geometrical center of the water-block base plate, an artificially induced bow in the water-block base plate increases the contact pressure at the geometrical center of the IHS and can be further qualified as a pressure gradient applied to the entire surface of the CPU IHS with a maximum pressure value at its geometrical center. The observable result is an improvement in the thermal joint between the CPU IHS and the waterblock base plate in the general area directly above the CPU die(s), while the water-block thermal resistance remains approximately the same.
Now the problem with a bow is when intel changed from solder to paste.
It now allows the IHS to shift geometry or in a sense move ever so micromil and not apply even pressure on the bow.
Also there will always be idiots that delid the cpu entirely and decide to put a bowed block ontop of a naked and die, and well, 5 letter word.. I D I O T.
But bowing also makes the concave heatsink moot, as it again applied enough pressure to fix the deformity of the heat sink.