According to Ina Garten, she says the pit has nothing to do with it...
I've always cut my avocados early in the day, put them on the bottom of the salad bowl and cover with the lettuce. They keep just fine until I finish the salad at night. I buy my avocados hard and let them ripen on the counter. If I use 1/2, I wrap in plastic and they keep for the next day just fine. I think the trick may be to use them at the perfect ripeness. If they have any soft spots or have the least bit of brown on them anywhere, they turn fast.