This would have been a more interesting poll if you were asking about split-screen preference. Anywhoo horizontal, both for consoles and split-screen.
That's what I thought this was going to be at first but alas, think we've had this question before too. PS3, Wii - Vertical because it sucks in discs. 360 - Horizontal because of the disk tray.
If my game systems were not horizontal then they would be covering part of my screen. Though now that I have a wii, I don't really like how close together all of them are. Not enough room to breath with them on their sides. I feel like they might attack each other any minute. Guess I should buy a new entertainment center.
Vertical. Everyone says horizontal is better, but from experience, that's not necessarily true. Two 360's I bought in the past broke as they sat horizontal; I have the current one vertical, and it's been good for years now. I've even after a long while set one of the "broken" 360's up again, and set it up vertical, and what do you know, it works! I don't use it though, but still, all systems are vertical from now on, unless a company says that I have to set it up one way or the other. Also, it's the way I have it set up because it's the best way I can do it for how I have my consoles placed.