840:
Interesting idea about the LCC A319s long-haul, but I'm not sure. First, I believe there would be a range issue with full load. Second, the economics. Willy Boulter of VS gave this example at the Future of Air Transport conference in London in November: over the winter (he said), LHR-JFK round-trip fares are typically as low as STG199 including taxes. Taxes and surcharges are typically STG113 return, giving a one-way net yield to the airline of STG43. (WB said, if I understood him correctly, that the cash costs of operating the sector - fuel, food and handling - were of the order of STG45 per pax. His argument was thus that it isn't really possible to bring prices any lower as these kinds of fares are already "loss leaders" and only possible because of the higher yields up front.)
OK, so maybe a one-way transatlantic yield of say STG43 is currently the worst case, but clearly extra capacity in the market would drive prevailing yields down rather than up. And if you consider that an LCC will manage typically one transatlantic round-trip per day (or 1.5 if you start scheduling very aggressively and at antisocial times), that's fairly slim revenue pickings per day compared to eight-to-ten short-haul sectors daily (and that's before considering the cost implications of long-haul).
You're right that the LCCs will be looking for areas for expansion with their A319s, but I think long-haul with LCC A319s will be a LONG way down the road.