This is an interesting topic, I've always wondered the same thing, regarding the accuracy of the readings on these machines. I have both a treadmill and elliptical at home, and I tend to use the "calories burned" indicator as a motivator, not as guaranteed accuracy.
I have been putting my activities in Fitday.com and cannot find the elliptical machine as an option. I looked under general exercises, and then tried to search, and still couldn't find it. Does anyone know where it is? It helps me to keep track of everything to ensure continued progress!