I'm in a similar situation. I've been waitlisted at my top choice, yet I have a strong offer elsewhere. From what I understand, they are probably not going to get around to the waitlist until after the 15th. One of my professors suggests that I wait until the last minute to accept the other offer, then if I get into my top choice, I may decline the previous acceptance. Personally, I really don't like that way that sounds...
One hypothetical: Some big name school puts you on a waitlist however you have another school that has given you a good offer. The "ethical" thing to do (if you really want to go to the big name school) is to decline the other offer and wait around to get called. Unfortunately, this makes no sense because there is a decent chance that you won't go to school at all. Therefore, the ONLY choice is to accept the other school's offer and then decline it if the big name school comes through. Unfortunately, this may burn a bridge, but keep this in mind: the big name school is forcing you into this position. It's the name of the game.
Thus, we have several options:
1) Accept UCSD and forget about Stanford.
2) Accept UCSD and if Stanford calls, revoke your acceptance to UCSD.
3) Decline UCSD and possibly get into Stanford (although you could loose out on an entire year).
4) Take the year off, get some more experience and reapply next year.
5) Forget school for now and start a new life as a dairy farmer.
The choice is yours. Personally, I'd at least wait until the 14th. Honestly, I hate the whole waitlist process and I wouldn't wish it on my worst enemy. However, the process is almost over and you'll probably get a decision in the next couple weeks. Until then, contemplate option 5 and hopefully you'll realize that its not about the school, but its about your educational and professional development. Best of luck, hang in there!