I don't think your head unit's usb is meant to be a charger. I don't even think you'll be able to charge the phone with it. You can try to plug it in and see if it begins to charge, but I highly doubt it will. If it charges, then Yay you've got yourself a charger. I'm sure it won't break the phone or the head unit, unless it doesn't have the chip that will stop charging the phone once it's fully charged (which will kill your battery's health). If it doesn't charge, oh well it was worth a shot.

 

My Pioneer stereo has a USB and it charged the iPhone no problem. Unfortunately it doesn't charge the Droid, nor does it recognize the SD card.

 

Your phone charges off of your laptops USB port...right? Your computer doesn't have a chip to stop charging the phone when its full. A lot of devices nowadays will stop the charge themselves rather then rely on the charger to control it.
A standard USB cable has 4 pins, power, ground, data+ and Data-. This does not change from cable to cable or device to device. The power and ground will be the same pin whether its going to a Ipod, samsung phone, or htc phone. Hook your phone up to the headunit, and maybe every once in a while make sure that its not overheating the battery, and I think you'll be fine.

 

Here is an idea...plug it in and see if it works