The problem with the 4,1 DP machine is the way the processors are held in place.
The 4,1 is designed to use CPUs that don't have the integrated heat spreader. Those CPUs were only used by Apple and are 2mm shorter than the XEON processors sold to everyone else.
If you try to use a CPU with the IHS, then you end up crushing the CPU and/or the daughter board when you tighten down the heat-sink and fan assembly.
So I don't think we'll ever see a flood of cheap CPUs that are direct fits (unless they're from other 4,1 Macs).
We also are limited to Nehalem CPUs because of the 4,1 firmware.
But that's not the entire story.
There is a way to upgrade the 4,1 firmware to the 5,1. That allows you to use the Westmere processors.
And if you're brave, you can install a CPU that has the IHS. You just need 2mm washers to raise the heat-sink in order to accommodate the extra height. You also have to modify the power cable for the fans. The extra 2mm lift prevents them from making contact with the daughter board.
The 5,1 doesn't have any of the problems I mentioned. You can already find cpu upgrades form Newegg.