Grumpy_Mike:
If the two sub systems can be independently powered then a seriese resistor and catching diodes to each rail are a must.
Is that considered ESD protection? I thought the inputs already had this anyway? (Honest question -- I'm not that advanced of an engineer, and would love to know what would make for good design practices.)
Grumpy_Mike:
How can a USB device be plugged in and not powered?
Well... How many desktop-sized drive enclosures have you found that are bus powered? 
(In my case, I'm talking about -- for e.g. -- a Plextor half-height optical drive in a USB-to-IDE enclosure.)