I've played with a few USB car and AC chargers to find out what might cause GPS loss when charging. All the chargers will charge and keep GPS with Android (Motorola) and showed AC Charging (which is correct the pull higher current) not USB Charging. But only a few will work with N9 in charging and GPS.

Using a charging only USB cable (2-pin shorted) is guaranteed to lose GPS, even though it will charge, you must you use a regular data cable to keep GPS.
(tested on a few AC chargers that will charge and work with GPS on N9, as soon as I changed to any charging cable I have, it dropped the GPS)

So, probably many of those USB chargers which simply shorted 2 pins will charge N9 but will lose GPS. There has to be something more than just shorting 2 pin to keep GPS working.

I finally found one CHEAP USB car charger that can charge the N9 AND keep the GPS, T-Mobile white usb car charger for HTC MyTouch $2.75 shipped from Amazon (1000mA)