Does every floor contain a secret room (entered via bombed wall or having X-Ray Specs), including the Womb and Sheol?
I can personally confirm that every floor from the Womb up has secret rooms. I made it into Hell only once, though, so I don't know about that. I'll see if I can find anything to back up my statement.
EDIT: No 100% confirmation, but looking in the Steam forums, it's taken as a given that secret rooms show up in the Womb. http://forums.steampowered.com/forums/showpost.php?p=28869410&postcount=7
Yes. The Secret Room is a special room type. One secret room will appear on every floor in the game and will have a pre-determined location when the floor is spawned.
The Secret room cannot be entered normally; ways to enter include:
XXVIII The Moon Tarot Card Random Teleport Bombing in through any adjacent room (excluding boss rooms and mini-boss rooms) Using the X-Ray Glasses
Ways to identify the Secret room on the map include:
The Treasure Map The Spelunker Helmet (when in an adjacent room) XXI The World XIX The Sun X-Ray Glasses (when in an adjacent room) The Compass (will be identified as a mini-boss room if and ONLY if Greed is in the Secret Room)
The secret room is typically placed adjacent to three or four other adjacent rooms, often adjacent to a Treasure Room if possible. If applicable, the Secret Room will always have preference over three adjacent rooms to two rooms. This preferrence does not occur when comparing three adjacent rooms to four adjacent rooms.
v1.48: Yes & No. On rare occasions only the Top Secret/Hidden Room will be generated, due to the specific rules used for Secret Room placement. So there will always be a (Top) Secret/Hidden Room but not always a Secret Room, except on the first two levels Basement/Cellar where the map will be redrawn until there is a Secret Room.
I've had this twice personally when I had enough bombs (30-99) to bomb every wall. If you've found the TSR/HR, and the SR is not next to it and is not in any location that borders at least two rooms, you can stop there, as the Secret Room is never placed such that it borders only one room (only TSR/HR is).
If you have a Moon card, using it in this situation will teleport you to a random room instead of the Secret Room. Also there are various items that reveal the location of the Secret Room when you possess or activate them, if there is no Secret Room then one won't be revealed.