My brother, a brewer for Summit in Minneapolis, sent along this video as he knows I am obsessive about cask ale. Note the process at a big production brewery like Summit: prime the cask and then add beer from the conditioning tanks. It is in this step that the run afoul of the CAMRA types as far as I understand - true cask beer is not primed but finishes out in the firkin.
Nevertheless, to me the main distinction is the lack of injected CO2 that changes the flavor profile dramatically.
Brewing TV - Episode 36: Gone Firkin from Brewing TV on Vimeo.