First of all, don't just take the fan out of your PSU or anything. That's not good, and a PSU failing can take out other components with it, such as CPU, motherboard, video card, etc...
Watercooling a psu has been done at least twice that I know of. Both were members of HardOCP's hardforum (it's down right now, so I can't link to it). Not very easy to do, but yeah, it's been done.
I think just buying a passive PSU would be best if you need dead silence. I would, however recommend a Fortron 300w PSU with "noise killer". It's got a big, slow running, quiet 120mm fan. And don't let the fact that it's "only" 300 watts slow you down, not many single processor systems use that much power, and these things have been known to put out 380 watts with no problem (an overclocked PSU I guess).
As far as the point of silent systems goes, I guess it just depends on the person. I hate having a rig so loud I have to turn the TV up just to hear over it. I couldn't (before I silenced my rig) talk to other people in the room if they're on the other side without raising my voice a tad, which is annoying. Plus, this is a little more rare, but some people have specific needs. For example, we have a two room recording studio in our basement, and the audio encoding/recording computer has to be pretty silent.
Given it is in a control room, which is simi-sound proof, but it's nice to not have to worry about opening up the window and screwing up the track because of the noise of dozens of fan spinning away...
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