Hmm...There is a good question.

48V would certainly have some advantages. But you might have some issues with regards to the -12V that is still usually provided for legacy RS232 use. At that point you have 60V (nominally) of potential difference between 2 points on the motherboard. Given that most PSUs today are switching supplies and that unloaded voltages usually exceed specified loaded voltages, it is easy to see that we would exceed the 60V 'Low Voltage' classification, hence requiring greater safety precautions. Given the fact that such a large percentage of PC users open their PCs and upgrade stuff themselves (many of whom are, quite frankly, clueless about electricity) I'm not sure that this is so desirable.

On the other hand, ATX is getting a bit long in the tooth. There's too much provision for legacy whilst too many upcoming technologies are battling for space. I think that it's nearly time for a new form factor to become prominent. But I don't think that any of the existing alternatives have got it right yet. Perhaps a new form factor specification would eliminate RS232 (in favor of USB dongles when necessary) and include 48V supplies.

But I'd quite happily see 24V used as a compromise.