According to a friend, the origin of this expression is in the Yiddish 'Hatsluches un Bruches' meaning 'Success and Blessings' (from the Hebrew 'Hatslahot ve Brachot'). Apparently, the Germans heard this used often by their Jewish neighbours, have germanized it into 'Hals und bein brochen' (I'm not sure about the spelling), meaning 'Break neck and leg', which was converted in turn into the English phrase 'Break a leg'.