[Tronz]

No need to carry those large power supplies around anymore ( If you are running the Rockbox firmware ). Simply plugging a USB cable that's already plugged into a powered USB port into the DATA EXC port ( Not USB DAC port  ) will charge the HM-801. There are 2 downsides to this. 1, the USB method of charging requires more time to fully charge the battery than the supplied AC power supply. 2, the HM-801 is useless if you try to charge it using this method with the original firmware. With Rockbox installed, the HM-801 doesn't enter a " USB DATA EXCHANGE screen" when you plug a USB cable into it that is currently connected to a computer. You can still use the device, but it's now being charged. Also, I tried to see how far I can go with this method by removing the battery from the HM-801 while it was receiving a charge from the USB cable. With the AC cord connected, the HM-801 will continue to function completely if the battery was removed ( Some say with improved sound quality ). If the HM-801 is being charged via USB and the battery is removed while music was playing, the device remains on, but the music stops followed by a medium pitched tone. The reason being is that USB which is limited to 5v 500ma is not enough current to power the HM-801 for musical playback.
 
 

USB cable plugged in and charging the HM-801's battery. 
 

HM-801 running off of internal battery. Notice the battery icon.
 

HM-801 running off of USB power only.