new micro usb charger standard say that data lines ( pin 2 & 3 ) must be shorted for detecting charger. this is imposed in 2006 by chinese government and adopted now wide on the industry. and is part of the usb2.0 standard.
n900 charging circuits are smart enough to detect computer wich can provide 500 mA or high power charger up to 1.8 A ( data lines shorted )
google is your friend .