I suppose you could always pop the unit out, drill a hole through the facia and then wire up an input to the circuit board inside somehow (ie. through CD drive signal terminals). Another option is one of those dirt cheap wireless transmitters that you can pick up from some petrol stations for only £8 (got mine from Shell). Of course, for £40 you can buy yourself a whole new headset and then fit that, flogging your old one for 10-20 quid if you feel like it!
-Pos