[CliffordK (OP)]

I was making some eggs today in an electric egg poacher, and started wondering what causes the air bubble to be in the big end of an egg?

I suppose the air bubble is also visible in freshly cracked eggs, so it is not merely positional while boiling.

Traditionally we store eggs in egg cartons with the fat end up. 

So, if one took a very fresh (less than a day old) egg and stored it skinny end up, would the bubble form in the skinny end?  What about storing it on the side?

[damocles]

...{snip}...
Traditionally we store eggs in egg cartons with the fat end up. 
...{snip}...

Here in Oz we traditionally store eggs in egg cartons skinny end up, but the bubble is still formed at the bottom!
I think that the answer has to be that there is much more structure in an egg than just a white and a yolk -- in particular a membrane around the bubble attached to the shell.