on the other hand, i've seen cases where an old 12 Volt fan or a crappy harddisk either shorts out or worse induces a lot of noise into the line.
With a power supply per server, this usually affects only that one machine. Now, if you have a power supply per rack, this might affect all servers in that rack. Especially with the noise-on-the-line problem, this might not actually shut down or crash the servers, it might make them unreliable -> much harder to find the bad component!
I'*d say, rather than changing the power supply, we should focus on the power consumption. From what i've seen, most servers in a company are completly over-sized in terms of capabilities because "at some point in the next 5 years (well after the next upgrade...) we'll need that resources". In many of these cases, the companies could save a lot of energy and money by buying slower and "greener" servers without really reducing service.
In one case, i've seen a top-notch AMD Quad-Core Dual-Processor Machine with 8 Gig RAM used as an Email-Server for about 10 people. And yes, it also had a top-notch graphics card, too. Hooked up to a DSL-Line. In this case, a 5 Watt Soekris would have been enough and it probably even would not have slowed email transfer...