Ok, this is one for the guys.
I don’t really care about whether I leave the toilet seat up or down, except that I leave it down as a courtesy towards the ladies in my family. But the toilet seat lid… oh ho! is that a different matter! In my house, if you’re not using the john, the lid stays DOWN.
So what about you? Are you a lid-up-and-smell-the-flush-plume kind of man, or are you of the lid-down-like-any-civilized-person sort?
The Dun King
Seat down, lid closed. If I have to lift the seat to pee, the lady has to lift the lid to pee, gender equality and all. And it keeps the cat from falling in.
Lid up, definitely. Provided you keep the bathroom (and toilet) clean, the lid serves no purpose whatsoever to a person with no pets.
-FK
Lid down for me; my wife leaves it up (the lid, not the seat too, obviously) and it annoys me (I probably should tell her it annoys me, but I just close it and fume impotently, thinking that she will notice my impotent fuming and deduce the nature of the problem).
Lid up. The cats appreciate it.
Everything closed. You see, I have a toilet seat cover that’s decorated like a fish. (Don’t ask.) You can’t see it unless the toilet is closed.
FISH
Lid up. The dogs think it’s their own personal water cooler, and the cats like to watch the water swirl when you flush. (They’re pushy and know how to open doors, okay?)
Um, hate to break it to you but Cecil did a column on this: Does flushing the toilet cause dirty water to be spewed around the bathroom?
As for my place, the lid’s broken and always stays down unless held up (yes, this does make it more difficult to pee, and yes, I’ve gotten over it). Also funny when friends are over, going WTF?
I live alone and lid down definitely. Lid down before I flush, for the reasons stated by Unca Cecil.
Lid and seeat down (even the ones in public bathrooms with open seats), and I also live alone.
You see, when I was a young teenager, my family had a parakeet that was never caged. One night my family came home after dark, and couldn’t find the bird. Eventually we found that it had drowned in the toilet. Very sad. We loved that bird (Jo Jo).
Even though I realize that this is rather irrational, the urge to close the seat and lid are very strong. I even do this in other peoples bathrooms. Habits can sometimes be weird.
Bob
Oh, yes. I should mention that that happened almost 50 years ago. 
Bob
Seat and lid down for me, as we’ve pets and kids and I just like the look of it better. My wife is not quite as regular about it as I am, but that’s what I do.
Lid up. I don’t think I’ve ever put the lid down…
Seat down, lid closed. I started doing it because I misunderstood the whole “seat up vs. seat down” debate when I was younger, and it became habit. Cecil’s column only strengthens my view that I’m doing the right thing.
Dun King:
Lemme see if I’ve got this right – you don’t care whether the seat (the ring-shaped thingie that you sit on) is up or down, but the lid (the flat thing that covers the hole in the ring-shaped seat and caps off the whole works) must be down. So, umm, as long as the lid is down you don’t mind if the seat is up? Perhaps you could draw me a diagram?
Er, yeah. I’ll forgo the diagram, this isn’t all that complex.
I don’t care about the seat, I care about the lid. I don’t worry about putting the seat down if there is no lid (e.g. in public restrooms or the odd private latrine without a lid).
I’m less concerned with splashing on the seat (less splash onto the seat if it’s up, right?). I don’t like seeing the inside of the bowl, though, since I don’t clean it every day. That, and the pets think it’s the most wonderful, flavorful water dish they’ve ever seen. (Then again, they think that any time you put ice in their water dish…) It looks neater, and serves to ease my nerves about that dang flush plume.
The Dun King
For some reason, I like to time my urination with the flush. In other words, I push the handle while I am still pissing and hope that in the 4 seconds it takes for the water to swirl and drain away I will have squirted my last drop. Currently I can time it exact at an 80% clip.
And then I put the lid down.
I actually find it harder to lower the seat without lowering the lid…
Until I read Cecil’s flush-plume column (scroll up for link) I left both up (I live in a college townhouse with 3 other males and have my own bathroom anyway) but now I drop both.
I may have missed the reference, but I didn’t see anywhere, that putting the toilet lid down lessens the spread of germs. I have however added several bottles of bleach to the shopping list.
This is a prime of example of the Straight Dope changing the way you live your life. After reading about toilet aerosol here I now close the lid BEFORE flushing.