You've probably heard from your parents that a waitlist is, for all intents and purposes, a rejection. However, at CMU, it is not necessarily so. I was accepted through the waitlist, and I know 5 other people who were accepted. Out of a 2008 CS class of 130 students, most of which I don't know, that's already pretty good odds. I don't know any statistics for specific colleges, however.
Carnegie Mellon has two waitlists: the priority waitlist, and the regular waitlist.
Priority waitlist: This is like an "early decision" deal, except not. If you are accepted through the priority waitlist, you are expected to attend CMU. According to the letter, the odds of being accepted through this waitlist are around 50%, very good odds indeed. Note that this option is ONLY available to people who applied to one school in CMU and was waitlisted. If you applied to more than one, you will only have the regular waitlist option. As the name suggests, you have priority over other waitlisted people. Spots that open up will be filled from this list first.
Regular waitlist: This waitlist is what your parents envisioned when they told you it was pointless. If you applied to more than one school at Carnegie Mellon, then you will not be allowed to use the priority waitlist (I believe even if you were rejected to all except one). The odds of being accepted from this one are much less, but it could still happen. If you are dead set on attending CMU, then choose this option.
There are also a few benefits that come with being waitlisted. Since you get housing after everyone else, you will usually end up with better housing. How? You get what's left over from the upperclassmen that decided to move out during the summer. For example, out of the 5 waitlisted people I know (myself included), we got a Mudge single, West Wing double, Resnik double, and a Morewood triple. That's some of the best housing on campus. And hey, if you don't get housing right away, you'll get to live in a hotel until they can find you something.