DD asked me the other day where the toilets were in the TARDIS. One of those things I'd never given any thought to. (We used to see some of the other rooms in the old series but so far since the revival we haven't. We don't even know where Rose/Martha/Donna slept.)
I think "Babylon 5" is the only sci-fi show where I have ever seen people actually using the facilities. Somehow you can't imagine it in "Star Trek" - Captain Picard excusing himself during the Borg invasion for a quick dump (no doubt the doors would still do that psssssht! thing) and emerging after a few minutes to say to Riker, "I'd leave it for a little, Number One. That's what I call a Captain's Log." And Deanna Troi would smirk a little and try not to make any jokes about Klingons.