The aphorism, "two nations separated by a common language", has been attributed to various notables: the toss usually being between George Bernard Shaw and Winston Churchill; although Oscar Wilde, Bertrand Russell and even Dylan Thomas also get a look in on its provenance. The common literary consent, although it can be found nowhere in his written works, is that the nod goes to Shaw. And whilst some of the perennial old chestnuts (fanny-pack/bum-bag, cigarette/fag etc.) are widely known, to say nothing of still smirked at, there has, of late, been a fresh batch to add to the canon.
In either case, as to appropriate usage, it is still very much the advised rule of thumb of 'when in Rome'; it's all very well waltzing into an American hairdresser's and asking for 'a shag' (to them a hair style), however the response from its UK equivalent may well see you leaving the salon with the fat lip or uncut hair - or probably both.
In the US, a 'Town Hall' is now a meeting (although, for some reason not immediately apparent, the 'meeting' qualifier has become redundant and superfluous to requirements - a 'town hall' in the UK still being, well, a town hall) - worth noting, too, that in the UK, to make a 'town halls' of something is rhyming slang for making a 'balls' of it.
In fact one of the things which has lent English (be it Am.Eng. or Brit.Eng.) such longevity, apart from its magpie ability to loot, pilfer and steal from whichever language it chooses, is its unrelenting practice of adapting to accommodate new words, uses and constructions; a comparatively recent case in point - and, to some, a somewhat hastily thrown-together hyphenation - being the new American verb, 'to man-up': meaning to be in possession of sufficient quantities of testosterone as to enable one to face up to, and accept, one's responsibilities (cf. 'have the balls to do'), as in "Sarah Palin has yet to man-up and announce her candidacy for the 2012 presidential election".
And, whilst we're on the topic of testicles and places of civic meeting, habitués of US 'Town Halls' may also be referred to as "Teabaggers" - to some, an otherwise perfectly respectable act of gross indecency, requiring a chap to dangle his undercarriage in the mouth and across the face and forehead of his beloved (or at least one for which he may have paid for the privilege of being allowed to do same).
Any way, given the apparent duality of the language, perhaps it's fitting that cross-dressing stand-up comedian Eddie Izzard have the last word on the subject. Enjoy!