Chagrin:
The acceleration due to gravity is pretty constant 8) so when you've got it sitting still in any position you know the value it's reporting is 9.8 m/s2. The datasheet for the chip states it is factory calibrated so there shouldn't be any second guessing required.
I'll be honest I attempted to read the datasheet but a lot of it seemed like jibberish to me.. I'll read it again and at least try to find the part you mentioned so I know for next time.. anything else I should know? A friend of mine suggested I look into "Azmuth equations?"