Hi,
As yet another UCSF student I just want to reiterate some very good points already made by citygirl and jtheater...
1. The "15th of the month" deadline is not a deadline or a hard-and-fast rule. I think it has gotten out over the years that acceptances are "always" sent out on the 15th and that's not entirely true. The 15th is a nice middle-of-the-month target to shoot for, mostly because it tends to fall a week or so after the committee meetings. That said, things happen. Meetings can run late, a person's particular application may not make the monthly schedule, and other unforeseen but generally non-preventable occurences often happen. And, as previously mentioned, you cannot discount the tribulations of the mailing process, especially given the current holiday crunch. So please, I know it's a major stressor, but a lot of factors come into play in determining which day, week, or month a letter may arrive in the mail...and most of them are not really controllable. Keep calling the office if you must, but please keep this in mind.
2. As for interviewees...as was previously very well put, you're in great shape overall. 50% or so will ultimately receive an acceptance letter, and getting an interview at UCSF pretty-much means you will be getting into a medical school, unless, say, you ONLY applied to UCSF.
3. As for holds, yes, a certain number will eventually be granted interviews. However, there's really no way to determine how many that will be or when the parsing step will occur. I realize it's very aggravating to be in this limbo-like state, but there's not too much to do except wait. Sorry.
But, remember, it's December...it's still very early on the game, although I know it feels like it's been forever. The majority of interviews has not yet occurred, if that's any comfort.
Just my two cents...