It all depends on the wiring standard thats used on the TRRS mini jack..
I think this is how it is on the X10 (and also iPhone, HTC, Blackberry etc..)
1 Tip Left Audio Out
2 Ring 1 Right Audio Out
3 Ring 2 Ground
4 Sleeve Microphone Input\answer button
I know Nokia have pin 3 and 4 swapped around so wont work with the X10, and there may be more out there but I think most are trying to standardise to the above setup.. so you shouldn´t have too much trouble finding a headset to work with the X10 (just stay away from Nokia)
(the extra bits next to the mini plug are just for powering the SE portable speakers and other SE crap you will be able to buy.. nothing to do with headsets)