While some may make the N900 wake up and attempt to charge, if it doesn't put out the right amount of amperage (my experience is ~350mA) the N900 will eventually note that it's using more power than it's getting and stop attempting to charge. You'll see this issue when trying to charge from a computer that's off, but leaves minimal power on to it's USB ports (which is typically at it's start value of 200mA), if negotiation fails for some reason, or if something is actively using lots of juice (overclocked/trackerd, etc).

Mind you, if you're doing anything at all on the N900 while in the car (nav, streaming media, mp3 playback, etc) it's going to want more power, not less. So even a charger that works when the device is idle may fail to charge when you're using the device.