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An incorporeal creature can enter or pass through solid objects, but must remain adjacent to the object’s exterior, and so cannot pass entirely through an object whose space is larger than its own. It can sense the presence of creatures or objects within a square adjacent to its current location, but enemies have total concealment (50% miss chance) from an incorporeal creature that is inside an object. In order to see beyond the object it is in and attack normally, the incorporeal creature must emerge. An incorporeal creature inside an object has total cover, but when it attacks a creature outside the object it only has cover, so a creature outside with a readied action could strike at it as it attacks. An incorporeal creature cannot pass through a force effect.
The rules state that an incorporeal creature can pass through a solid object, not a creature. Based on that, you cannot move into a square occupied by an incorporeal creature unless you have an ability that allows you to enter a creatures square. If that action would normally provoke, it would do so there as well.
Ahhh, the sanctity of our 5' squares...
Ever been in an elevator? The typical elevator is only barely bigger than a 5' square. A "square" in Pathfinder is 25 square feet. A standard size elevator with 2500 lbs. capacity typically has a floor area of 27.1 sq. ft. (about 4.5ft X 6 ft.).
How many people can you fit in a typical elevator? 8? 10? At least 10, if most of them are fairly lean and healthy.
So you should be able to get about 10 people into a 5' x 5' square.
So why does a Pathfinder human occupy an entire 5' square all by himself? Well, he doesn't. Not normally. If that were true, he could never sit in a crowded tavern, never walk through a crowded marketplace, never ride in a stagecoach, never use an outhouse, never sit in a rowboat, never sire any children, etc.
A Pathfinder human ONLY takes up a 5' square when he's in combat. Why? Because he's fighting! Attacking, thruting, slashing, sidestepping, ducking, bobbing, weaving, floating like a butterfly, stinging like a bee. Doing all that takes space. We need that space to fight effectively, and so does any other person who is fighting against us or near us - they don't want to crowd their way into our space because that limits their mobility. No more attacking, slashing, floating, stinging, anything. Too crowded. And we don't want to enter their space for the same reason. Too crowded.
So we all keep our distance. Distance codified in Pathfinder as 5' squares.
But, if you think about it, even if Jet Li is standing in an elevator with an open door, you COULD move into that elevator. He might beat the crap out of you, if he wants to, but you COULD do it.
And you could do it if it were Bruce Lee's incorporeal ghost in there, too. If you wanted to.
So why can't you in Pathfinder? It's not because every person in Pathfinder is actually a 5'square Lego person - they certainly don't fill their squares, even when fighting, so there definitely is enough room for two people to stand in the same elevator, even if they're fighting.
So why can't we do it? Rules. You don't want to be crowded, the other guy doesn't want to be crowded, so rather than building a complex set of mechanics for penalties to attacks, AC, saves, skills, etc., based on being crowded, Pathfinder just says you can't do it. Period. (Normally).
Being incorporeal does not change that. If you're fighting, you still need your 5' space for all that floating and stinging, and the incorporeal creatures still needs its 5' space too. Neither of you want to be crowded, so neither of you violate that spacing. Period. (Normally).
TL;dr: RTFP...
I'd say yes.