Can an incorporeal creature occupy the same space as another creature?

Question

Can an incorporeal creature occupy the same square as a normal creature? Does it matter if it's during a move (moving through another creature) or at the end of the move (stopping in the other creature's space)? How about another incorporeal creature, is that case different?

Several things cast doubt on the conclusion being obvious. The SRD says incorporeal creatures cannot be Tripped or Grappled, nor can they perform those actions. It also says they are weightless. When it says

In fact, they cannot take any physical action that would move or manipulate an opponent or its equipment, nor are they subject to such actions.

I understand that to mean Bullrushed and Overrun as well, even though the book only mentions Trip and Grapple as examples. So they can't use those methods of moving through an opponent's square.

It's also not clear if an incorporeal creature can pass through a square containing another creature without Tumbling (whether or not it provokes AoOs when it does).

All together, these leave open the question of under what conditions one corporeal and one incorporeal creature, or two incorporeal creatures, can occupy the same space, since that's usually disallowed except via one of these maneuvers or skill checks. It would seem strange for a corporeal creature to have no options to move into/through another creature's space, since it seems like the main point of incorporeality is to be able to move through things. Does an opponent provide an impassable obstacle for an incorporeal creature?

Can incorporeal creatures occupy the same space as another creature either during a move or at the end of a move, and if so, how?

Answer 1

Rules Compendium (p65), under the "Other Creatures" heading says:

An incorporeal creature can occupy the same space as a corporeal creature, unless the corporeal creature is entirely surrounded by a force effect. An incorporeal creature entering a corporeal creature’s space (or vice versa) provokes attacks of opportunity as normal for moving into another creature’s space. The entering creature must then succeed on a touch attack against the target to share the same physical space. If the target is helpless or doesn’t resist, no attack is necessary.

Emphasis mine.

The entry goes on to describe the effects of creatures occupying the same space granting cover or concealment, depending on relative sizes.

The RC mentions that WotC updated these rules originally in Monster Manual III on the same page (65), and those rules are reprinted in the glossary.

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What this means

Unless the incorporeal creature has another way to affect corporeal targets, such as the Ghostly Grasp feat (Libris Mortis: The Book of Undead, p27) or the phantom template (Monster Manual V, p130-1), it still cannot manipulate or physically affect corporeal creatures or objects. But it can share their space or move through it.

Answer 2

Incorporeal creatures cannot make trip or grapple attacks, nor can they be tripped or grappled. In fact, they cannot take any physical action that would move or manipulate an opponent or its equipment, nor are they subject to such actions. Incorporeal creatures have no weight and do not set off traps that are triggered by weight.

Monster Manual I pg311

Emphasis mine. No an incorporeal creature cannot bull rush. They will provoke attacks of opportunity as normal, but are immune to all physical attacks.

Two creatures cannot occupy the same space unless they are at least two size categories smaller (or one is helpless), there is nothing in the incorporeal entry to contradict this rule.

A creature's space is determined by its size. A large creature takes up a 10ft square, a small or medium creature takes up a 5ft square. An opening or window would allow the incorporeal creature to "remain on the objects exterior", so would allow it to move through a larger object.