You guys mean that yellow-tipped one, right?
That's the plug that was used for the GameBoy Pocket, the GameBoy Color, and just about any GameBoy Advance (not SP) power supply that let you disconnect the cable from the battery-compartment-clip (so you can use it on an older version of GameBoy).
(Two 1.5 volt double-A batteries = 3 volts)
According to those pictures of the innards of the E3 model DS, the DS's battery is 3.7 volts, and 860 milliamps.
A 3rd party GBA power adaptor I have sitting around (which works on the GBC) says that it's output is 3.6 volts, and 450 milliamps.
A 3rd party dual-mode GameBoy/GBP/GBC adaptor I have (which doesn't work on the GBA, says that it's output is 3 volts 250 milliamps when in GBC mode, and 6 volts 250 milliamps when in classic GameBoy mode.