I generally avoid chat shows like the plague, but I did catch one in which Hugh Laurie admitted that his American accent on "House" often slips in subtle ways. It's often more a matter of stress placement than actual pronunciation. He gave the example of "tomato sauce"; he'd nail the "tomayto" part, but would place the stress on the word "sauce" rather than on the "tomato".

This sparked a rather entertaining discussion between husband (Brit) and me (Canoodle) as to where the stress "belongs"... and whether it primarily refers to (1) ketchup or (2) that-red-stuff-you-put-on-your-spaghetti. (I seem to recall that when it's the latter, it is barely permissible to put the emphasis on "tomato" :-)