That's why POL products exists: http://store.mp3car.com/SearchResults.asp?Cat=10
I'm curious as to how one would go about wiring the power for the USB device to the car's own internal power. Wouldn't the voltage coming directly from the battery be too high for the devices (isn't USB 5v whereas a car is typically 13v?)?
Gen 1: Pentium 3 1GHz - ATX - 2005
Gen 2: Pentium M 1.6GHz - ITX - 2006
Gen 3: Pentium M 2.0GHz - 5.25" SBC - 2007
Gen 4: (coming soon: Core2 Duo - 3.5" SBC - 2009)
...it never ends
there are quite a few threads about how to power USB hubs....most of the time, using the 5v rail on your PSU is sufficient.