I personally thinks its better on the top hot air rises so why would you put one of the warmest components on the bottom to heat up the rest of the case also when the hot air rises the PSU fan helps extract it too.
I have the HAF 932. You'll definitely have to remove the large fan at the top rear of the case if you want to install the power supply in the top. If you install a 120mm fan further to the front it kind of defeats the classic bottom front to top rear airflow.
Ventilation, airflow, and cooling with the stock fans are outstanding. No problem installing the power supply in the bottom rear of the case.
Saiello - I have the Corsair HX620 in the bottom of my HAF 932 with the psu fan on the bottom so it can intake fresh air and cool itself. The top side of the psu never gets very warm to the touch.
I have the smaller HAF 922. The top fan I have blowing up and out, below it sits a Cooler Master Hyper 212 Plus with dual fans also blowing in push/pull out towards the top fan, the rear fan blowing in towards the the Northbridge heat sink on my Gigabyte motherboard and the front and side fans blowing inwards also. My Phenom II 965 BE idles at a cool 29C and on full load never gets above 48C...