Does every floor contain a secret room?
Yes. (I say this with the experience of always going to the secret room if I have bombs to spare, and having beaten Sheol dozens of times.)
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