What a great thread! I no longer feel so alone in the world... other people share my fear of bathrooms.
Actually, for me it's not so much the dark bathrooms, but going to the bathroom *at night* (even if the light's on). I don't like being on one side of any curtain, but I get most scared when I can't see the whole bathroom. So if I'm taking a shower, I get paranoid about what's going on *outside* the curtains (when I was a kid, I would force one of my parents to sit in the bathroom while I showered, or I'd even bring the cat in if they weren't available!).
And if I'm washing my face over the sink, I get scared the instant I close my eyes and have to keep them closed until all the soap is gone... meanwhile, in my head there are axe murderers, ghosts, what have you, all creeping up on me while I'm washing my face. Things got really bad when my grandmother died last November... I imagined her walking silently all over my apartment, especially when I was in the shower or washing my face. It took me a while to break myself of that fear.
I don't know where this fear of bathrooms came from, for me. I mean, I grew up watching horror flicks (thanks, Dad)... but I don't remember any particular bathroom scenes creeping me out. Maybe there's just something about the coldness of the bathroom facilities... so much chilled white porcelain, no carpet, etc. Dunno.