If you're dedicated enough to pay for the case then you'll make a good PSU fit
I'm AMAZED at the SFF PSUs that power up beefy desktop systems. If you can run a dual core 939 and SLI video cards in a SFF then surely we could find a similar PSU that we could shoehorn most anywhere?
I think a lot of the OEMS like Dell just use a different pinout on the ATX connector anyway. You can make an adapter or hack the plug up of a normal PSU and then it works fine with the Dell mobos. I'm not sure they still even bother doing this?