The dean at UCSD likes to tell this story about being accepted off the waitlist.
Several years ago, a woman (from CA) really wanted to go the UCSD, but was waitlisted. She was accepted to some school back in the midwest. She would call each week and then when August rolled around, she would call each day to find out what the situation was in regards to her position on the waitlist, etc. Each day the response was the same, "Call back later, you have not been selected yet". Finally, it was time to go to med school at the other school and she and her father took a couple of days to drive back east. Each night, at the hotel, she would call back to UCSD and get the same response.
When she arrived at the school and was getting ready to enroll, she called again and the last spot had openned up and she accepted and had to repack the car and drive all the way back to CA and enroll in UCSD.
I hope that doesn't have to happen very often, but it is an interesting story about the waitlist process.