It turns out that another reason that Into Dust by Mazzy Star (used for the Gears of War 3 adverts) is such an earworm for me was not that I had that track (although I do have a couple by them in iTunes) but more that I recognised her voice. The vocalist in Mazzy Star is Hope Sandoval who sings on a track or two of Heligoland by Massive Attack, most notably Paradise Circus which was used for the theme music of the Luther TV series. Also very earworm-worthy. Hope's voice is so ethereal and chilled out.
I am getting sleepy and so may head off to bed soon.