I've been thinking about this kind of thing lately. I have a 90W 12V DC brick and a DC-DC ATX PSU (very dinky).
Since the PC it used to power no longer needs it I'd love to run an Amiga from it but I suspect I'd only need 1 floppy lead and half a dozen pins from the ATX connector. If only I knew how to hack it.
Anyone else done this? Anything to watch out for? Do ATX PSUs get angry if you only use a few of their power lines?! I'd hate to burn out a pricey PSU and an Amiga.