Cue "Psycho" music when it comes to using Elisabeth Beucher's Green Warrior Shower Curtains. Beucher--part of the London-based textiles collective Puff & Flock--is one sadistic dame whose style philosophy of "design for pain and for our own good" aims to awaken people to the consciousness of their behavior and give them limits. Such as this shower curtain that slowly inflates (strangles?) around you while you shower, with the aim of trapping overly leisurely bathers. Claustrophobics need not apply.
Another take is a shower curtain embedded with spikes (picture after the jump) that will slowly inflate (via what claims to be an energy-efficient air pump) the longer you bathe. Overstay your welcome and you'll be spitted, though seriously these look more like ticklish tentacles than painful skewers.
Of course, there's always the concern that some people may get to like this too much. Still, either green-conscious shower curtain would assuredly ensure that family members take shorter baths in the morning, so you can get to work on time.