Heat distribution.

The hottest part of the Heatsink will be the center:

a) Because on most HSF combos the fan will have a 'dead spot' in the center where there is little airflow
b) the Core of the CPU will be in the center of the HS.

If I'm wrong, I'd really like to know why!  

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The peak temp spot varies on other factors as well as the fan location. Other fans, air flow, proximity to other heat generators or cables, and of course: foreign debris. Typically Ned would be correct, but one would still have to weigh the other factors. If I've got 4 DDR3200 modules next to the hsf on the right side and I've clocking on the edge of meltdown - right of center may be a bit warmer than dead center espcially if I have a side case fan pushing cool air directly on the cpu hs. Subject would be debatable I suppose...

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It will take me a couple of hours to get these numbers together and post them. I should have them up sometime in the morning.

Ned
I thought the same thing. My system proved otherwise. It probably starts out in the center, but it doesnt distribute evenly. My Cpu heatsink is the hottest at the bottom left corner. Im calling the Bottom the edge closet to the ground and the left is of course with me facing the heatsink. In fact it is about 9-10 F hotter than any other section of the heatsink. Im measuring the temp at the base of the heatsink where is butts up to the CPU. Something that totally blows me away is that about an 1" - 1"1/2 away from that spot and closer to the middle of the Heatsink there is a signifcant drop in heat. At least 3-4 degrees F. That seems strange to me. As for my case, it is a Thermaltake Shark. Its got 1 120mm intake fan on the front and down at the bottom. The only ohter fan is another 120mm exhaust that is of course in the back and about half way up. There are two fans on the PSU, but there is no way that they are causing the heat diff. Keep in mind that there is no direct airflow other than the Stock Intel CPU fan that is hitting the Heatsink. Even if you were to factor in the front intake fan, it would seem that it would cool the part of the heatsink that it hit first. The air thats coming in also runs through 4 IDE HD's before it gets to anything else. Im trying to be as scientific as I can to avoid the usual pitfalls (ie. confounding variables, experimenter bias). I have even added a blind side to the temp measurements so that the validity would be increased. I had someone that had no idea of what I was actually trying to figure out do the same temp measurements and they came up with almost identical results. I hope you have taken a statistics class, if not some of that may not make much sense. Like I said before, I was doing the temp readings for another little experiment that I have been working on. After I started to chart the different temps and locations I noticed how strange it looked. I cant answer your question about why the heatsink is not the hottest in the center. For all I know it could be as simple as the CPU fan cooling more towards the ends of the fan blades. If I can get the numbers crunched tonight, im going to let someone in the Physics dept. take a look at it Monday. I guess that would be a good starting place.

I thought about that, and it makes sense but it turns out that its the opposite edge of the heatsink on my CPU that is the hottest.

So, is that the point closest to the pwm (cpu power module, usually made up of mosfets, with each having it's own pi filter) I found that that was the hottest part of my mobo, at idle, and a close second under heavy load.
I used an infrared thermometer to measure. What are you using?