Pardon my ignorance, but what parts exactly do you guys think are going to rust from shower steam? Unless the bike NEVER moves from the bathroom, I'd think that any chain rust that develops would be disintegrated within the first few pedal revolutions of the next ride. Any chrome bits might develop some surface scale if the water is never ever wiped off, but if the OP's bike has mainly brushed/painted components it shouldn't be a big deal. Even then, chrome usually requires some serious moisture to develop rust -- more than the light fog that hanging in the bathroom would deposit.

Don't forget guys, the biggest culprit in rust development isn't water itself; it's the chemical ions that water carries with it. Unless homeboy is taking salt-water showers, it shouldn't really be a big concern. If it was, the metal faucets, shower curtain rod, towel rack, and toilet flusher lever in the guy's bathroom would've all disintegrated by now.