This is not possible for the following reasons:
- The phone can tell you its height above sea level, the accuracy of which has a larger margin of error than the difference between a sitting and a standing person
- Even if 1. did not apply, and you knew the precise height of the ground at your current location and the additional height of the phone, this would still be meaningless, as it doesn't take into account buildings, the height of the person, their posture and so forth.
You may have more luck using the motion coprocessor on newer models, you could assume that a standing person moves about more than a sitting person, or something. Or accelerometer readings to detect changes of position. But altitude is definitely not the way to go.