Rather interestingly the range of all these planes is converging (on the SYD-JFK distance, what a coincidence). The 777-200LR, 787 and now the 747-8 have similar range (wonder how the economics of the 747-8 will compare with the 787?).

Obviously DFW only makes sense for business traffic. Tourists would much rather route via LAX than DFW, for obvious reasons. It would be very foolish to run a high-Y plane DFW-SYD and I'm sure the airlines have this well figured out. DFW is desperate to attract long-haul international flights ... they built a new "international" terminal and 80% of the flights using that terminal are domestic (as there just isn't much international traffic at DFW, unlike ORD).