Isolation: if you connect 2 boxes with USB or with Firewire, you make an electrical connection between the 2. No difference between these protocols on this aspect.
True. For a long time, my preferred interface between the server and the DAC was via optical Toslink. I preferred the poorer interface over the electrical contamination.... until good USB/Firewire DACs started coming coming available in the past 2 years.
The only product I know that explicitly addresses this is the HRT Musicstreamer. This "isolation" was in all their marketing when they released the first product.
When I mentioned this to Daniel Weiss 2 years ago, he looked at me like I was some kind of idiot and said "if we didn't take care of electrical isolation in the studio, you'll get noise and hum." I guess it means that it's also taken care of in his products. So I've always taken it for granted.