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. What decides a creature space? 5 feet for medium? Can they pass through 5 feet thick walls, but not 10 (or 6)? What if the wall has a tiny opening/window/hole/tube? Can they squeeze through that? (perhaps does it count as surface facing?)
I'd like RAW and RAI answers and reasonable (based on balance or enjoyment or experience or whatever) house rules too.
Note: It appears Libris Mortis on page 140 has clarification on rules for Incorporeal, and arguably some extra rules too.