it was created for the advert by saatchi and saatchi, by a chap called ross gregory. i tried to get it recently but failed. i think the tune is called fibre optic, but as far as im aware, it hasnt been released. if you like that kind of music try philip glass, max richter or thomas newman tracks.
you could also try contacting lexus directly, i hope you have more luck than i did.
good luck hon