The glossary entry for
Phasing is rather sparse, and doesn't seem to address any specific issues that could arise while phasing.
Phasing: While a creature is phasing, it ignores difficult terrain, and it can enter squares containing blocking terrain, obstacles, and enemy creatures.
Presumably, floors and ceilings are blocking terrain, just like walls, so it follows that a creature with phasing would ignore those squares. However, if a creature without flying starts phasing, it would seem that the creature is no longer standing on solid ground and would begin to fall as normal, ie: towards the center of the world.
Therefore, it seems that phasing is more of a "selective" phasing, that it can ignore, rather than must ignore. Is this the case, or must phasing creatures ignore all terrain?
How does this interact with actually wanting to move through 3D space, though? Can a creature move diagonally up through a wall, selectively treating parts of the intervening wall space as stairs or a ladder?
Can a creature selectively phase part of it's body? Can it stick its head through the floor to see what's below, but maintain its purchase on the floor with its hands and knees?