AKL:
I should clarify that my 420s are "noise isolating" earphones by using the supplied foam ear plugs, as opposed to "noise cancelling" earphones which have battery powered noise cancelling circuits, but which tend to be less discreet and heavier.
Those Shure olives are excellent at blocking out noise, shame Sennheiser didn't put a larger pair of foamies in with the IE8's, as the single flanged large silicon tips are not so great at isolation (but are the best sounding for me), all the other supplied tips are to small or just plain odd shaped for me - I may try some of those Comply foamies from Westone some time.