Edit: I think I found a good answer below. Even if natural forces could lift Mjølnir, the enchantment would not be in effect - getting beaned by a 40-something pound chunk of metal would still hurt, but it wouldn't hit as hard as it does when Thor uses it.

~~Ah, but Magneto's not the one lifting the hammer - he's directing magnetic fields that are doing the lifting!~~

~~So, could the wind pick up Thor's hammer, if it were strong enough? How about changes in gravity - is Mjølnir as hard to pick up on the moon as at the surface of the Arctic ocean (Earth's highest-gravity location, I couldn't find coordinates)?~~

~~If so, then I ask: is the magic of the hammer smart enough to know the difference between a primal force and a primal force that's doing someone a favour?~~