Its not the power supply itself, its the noise that
hardware on the
powersupply creates.
I wonder if that's a factor that most power supply designers consider.
Certainly Zalmann are trying to sell their new silent case with very
low EMI as a feature:
http://www.quietpc.com/uk/tnn500a.php
Moving the A/D converters outside the main case might have a greater
effect than PSU design though.
Cheers
Daniel