From all of the threads that I have read concerning the placement of the PSU, and some high school level science, I feel that I have come to a decent understanding of why someone would put the PSU at the bottom. I also want to comment about cases that come with a PSU versus not.
Working on the base concept that heat rises, one would first hesitate at the thought of having the PSU at the bottom of the case. Logically, why would you want warm air coming off of the PSU and rising across your components? As I understand it, the PSU sucks air from inside the case and blows it out of the case. This would cause the heat of the PSU to be blown out of the PSU, sucking air from the bottom of the case.
My next point is if you are concerned about the placement of your PSU, you probably are building a decent machine. Otherwise, you would have gone to DE11 and just got a desktop computer for surfing the internet.
Following that thought, of why you are even concerned about the placement of the PSU, leads me to believe that you are going to be building a more powerful machine. This being said, you will probably be buying a more powerful PSU. The point of the PSU is not to cool your system, but to rather power it. That is what the other case fans are for. Being that you are using a more powerful PSU, and it isn't it's job to cool the case, you would want the PSU to keep itself cool and not contribute to the problem. If you had the PSU at the top of the case, knowing that heat rises, the PSU would not only have to take care of itself, but all of the heat generated by the other components, which are requiring more power, thus more heat...
The next step in this logic. You are building a more powerful machine, which probably means that you are putting in larger PCIe x16 cards (one, two or even three). The sheer size of these, and the fact that they have their own fans for the GPUs tell me that these are a major factor into the heat of your system. Also, these boards are towards the bottom of your motherboard. I can logically see hot air getting trapped under the video cards, seeing as air is being blown in from the bottom front, and sucked out at the top rear. Having the PSU at the bottom would logically help pull air across the video cards, giving the hot air somewhere to escape. Now, this does place some of the heat from the video cards onto the PSU as a burden, but at least it is not all of the heat of all of the components.
Coming close to a close, those who claim that the PSU at the bottom changes how much dust gets into your computer may not have thought this through fully. Maybe the dust collects more at the bottom, rather then being spread across all of the components. Maybe this is a more efficient way of moving air in your case, causing your system to pull more air through. I really have a hard time believing that the position of your PSU in your case has anything to do with how much dust is in the air. I mean, the thought that having the PSU at the bottom of the case causes more dust follows the same thought process that maggots came from meat (High school biology lesson). Besides, you should be cleaning your case monthly seeing that static electricity is the #1 killer of PC components and dust is the #1 cause of static electricity (humidity also).
The last point concerning the PSU and heating, I have seen it argued that having the PSU at the top near the other drives keeps your heat creating components all together, and that being a problem. I don't see this as being an issue, because if you care where your PSU is, you probably have good video cards, and having it at the bottom puts it near those.
The last points concerning the PSU position that I have found substantial is that it makes the case less top heavy (which I could see as a problem in a full tower case) and it makes managing the cables much easier. (A big plus, considering having your cables managed better would help improve airflow through the case... instead of a rat's nest slowing things down).
The one significant complaint I have found about having the PSU at the bottom is the distance between the power supply and the power connector on the board. If you buy a case that requires the PSU be at the bottom, I suggest getting an extension cord for the power. I found a good one on a popular online computer parts store for $6.
Lastly, those who say that cases that don't have a PSU upon purchase are $**t haven't thought this through. Most PSUs that come with cases are generic ones that probably don't have enough power. If you care enough to worry about where the PSU should go, then you probably shouldn't be using the PSU that comes with a case. Now consider two cases that are the same price. One has a PSU the other does not. I would think that the one that does not would be a higher quality case for two reasons: The people that made the case understand that anyone serious about building their own machine will probably buy another PSU; and there was more money put into the construction of the case, rather than filling it with a generic PSU.
All this said, I found a great case. The Antec twelve hundred. It has a place for the PSU at the bottom, and at the top, where the PSU would have been, is a large fan, facing up, not back. This makes a gaping hole in the top of the case pulling the air up and out of the top, where, knowing that heat rises, the heat would naturally float to.
Hope This Helps,
NPE (Null Pointer Expert)