Thanks all, this has been very educational. I think what happened is that China Eastern had some promo for a day where flights from Singapore to NYC were about US$1,500. However, I needed to connect a flight from Cancun to NYC after a side trip and Zuji, the Singapore Travelocity affiliate, just couldn't do anything other than enter flights into their own website and completely BS'ed me (it was at least amusing to speak to an Indian accented guy named "Brad"). The China Eastern flights are now gone.

I'm flying into NYC for something and decided to take a side trip in the Yucatan, which is why I need to stick the Cancun flight.

It seems that Air China had some promo for Singapore-NYC flights via Beijing (which unfortunately adds five hours each way!) and I took the advice and just pulled the trigger on Expedia. I got the flights for my entire 3+ week trip (counting an open jaw to Guatemala and another from Cancun) for slightly less than US$2k. I have now learned not to assume that all online agent search engines are equal, and Orbitz and Expedia are probably still better (although Asian agents may do it better for Asian regional itineraries).

Any more tips about online agents are welcome!

The original situation was that Zuji (again, the Travelocity affiliate over here in Asia) could get me the flights I wanted but only in two separate multi-city searches. Had I booked those as two separate itineraries with the same agent (silly, I know), how would you get American Airlines to send the bags to my international flight in NYC even though the flights are on separate tickets?