I just want to echo what BugByte said, that mp3car is very close on this one already. No need to reinvent the wheel here. I have a prototype unit installed in my car and I have had no problems. It works perfectly, so I doubt there is much more that is needed to get it to be sold. So hold off a bit, and the holy grail of car powered USB hubs will be here soon.
I actually wasnt overloading any of my USB ports before I installed it, but it gives me that warm fuzzy feeling inside that the devices are being powered from the car separately and not via the M2.
It's a great product-to-be, so just wait a bit.