I reworded "if you leave the level using the Bathysphere you entered with before you complete Cohen's Masterpiece when you return to the level: the curtain that needs to be open to perform that quest will be permanently closed.", "(Good luck decyphering that; I can't.)", to now say: "In Fort Frolic, do not leave the level using the Bathysphere that you begin the level next to. If you do leave Fort Frolic using that Bathysphere then when you return to Fort Frolic the curtain that needs to be open to perform the "Cohen's Masterpiece" quest will be permanently closed. If this happens then you cannot get to the Fort Frolic exit Bathysphere to progress to the next level."
On the official forums: http://forums.2kgames.com/showthread.php?110002-Known-Bioshock-Issues
Hope you're happy. ;) :p
headkase