For international flights, I've found that Orbitz SAYS they will let you book your seats online, and then DOESN'T. I have had to call Orbitz after booking and they've had to call the airline desks and add my requested seat assignments in manually.

They did DO it, however. At least, I think they did. We haven't flown on those flights yet, but immediately after I got off the phone with Orbitz I checked ExpertFlyer and the seats I had requested, which previously were marked available, were now marked unavailable. So they did SOMETHING .