We've got an 8 x 24 ASCII nude printed out in our living room, we've got seven or eight artistic airbrushed nudes in the halls that one of our two female roommates brought with her, and we've got one of our computers running a 500-image playboy screen saver at all times. We also have quite a few beer posters, a huge Ferrari banner, and some movie posters.
Our common-area poster/wall decoration/wallpaper/screensaver rule is basically no dicks, and no pubes. Except on Porn Nights.
Edit: just for demographics, we've got five people in our four-bedroom house, two girls and three guys, all within 2 years of 21 (girls are 19, guys are 21, 22, and 22)