With 5 wires connected to the Touch headphone jack (instead of 4 on the previous model), the device now supports the external microphone included with some headsets, according to Kyle Wiens of iFixit, who fully dismantled an iPod Touch on Wednesday.
Apple introduced the new iPod Touch yesterday which contained a couple of minor hardware tweaks over the previous model. Most notably, this included a built-in speaker and volume control buttons which were both absent on the 1st generation iPod Touch. Apple appears to have also added external microphone support (with an accessory) into the iPod Touch as well.