I believe the London-Denver flights are on hold. It is my understanding that United filed for this service when an open skies agreement between the US and UK seemed imminent, but it fell through nonetheless and now the flight is dead becuase UA can't obtain more LHR slots without the benefit of a bilateral agreement. But I'm sure the moment that UA gets another slot, DEN-LHR will be their first move. They really want to compete with British Airways into London. But for the moment all plans for DEN-LHR have stalled.
As for Mexico City service, I know for sure thats it's not opeating at the moment. Whether or not it's seasonal, I don't know. Now DEN may get MEX service from Mexicana. DIA execs have been begging them to fly DEN-MEX for quite some time. Not too long ago Mexicana said they would be pulling out of DEN altogether, but somehow someone convinced them to stay. Not only did they stay, but they added one stop service to MEX. Recently several upper management types from DIA flew on the one-stop flight to show the commitment DIA has towards getting MEX service. Mexicana has agreed that if loads are good enough, they will start non-stop flights between the two cities. But that wasn't too long ago so it will be a while before we see any changes.
As far as Frankfurt goes, that can't be far behind. With international traffic at DEN on the rise, and the Lufthansa/United partnership, DEN-FRA has got to be high on somebody's list.
All I can say for you DEN'ers is to be patient. DEN has never really been an international hub, and even now is just starting to develop. BA is doing great in DEN. With growth like that, airlines are bound to take notice eventually. But in an age like today where codeshares exist and costs are rising, it takes a lot more to convince an airline to start service than the excellent performance of one airline and propaganda from DIA management. It just takes time.