As a first year MD/PhD student here at UCLA I thought I might be able to give you guys some info. I'll try to hit all the points brought up, but if I don't please send me a note to ask more questions.
First, two years ago (when Vader applied) the program really screwed up, and didn't recruit properly or correctly judge the number of acceptances needed to fill their spots. That year's class had only 5 people in it. The people in charge were really upset and it's not going to happen again. Last year, however, they corrected things, and have a full class of 12 students. I was told that only 2 people turned down spots, so that means a total of 14 acceptances were sent out last year. I'm not 100% sure of that, as it's second hand info, but it was definitely less than 20 total acceptances.
I am not in the caltech program, nor did I apply to it, but the way it worked last year was that 2 people were admitted into the program. Those two people were first admitted to UCLA without knowing their caltech status. After accepting their spots they were then given a choice of whether they wanted to attend caltech or not. They may be changing that this year and telling people with the acceptance.
As far as cost of living goes, it's definitely true. It's expensive here, but it's comparable to the bay area schools, stanford and UCSF or the New York schools. And there's graduate student housing as well. It takes a year or so to get in, but it's very reasonable.
As far as stipend goes, it's 20K and we all hope it goes up. They pay for everything as long as you stay in good academic standing. Although, they do require you to become a California resident after the first year (if you aren't one already) so that your tuition is reduced. They also require you to apply for extra fellowships and grants during the PhD years. This is for two reasons: to help the program and because they feel that it's very important for students to get used to writing proposals. It also looks great on a CV.
Ok, I think that's most of it. I've been told that the total number of applicants and interviewees is up 20% this year, for unknown reasons. So, that'll make it a little more competitive. Also, we did have an 11 year old interviewee. I personally think that it'd be a little innappropriate to admit him, but we'll see.
UCLA has a great program that is only getting better. They really want to expand the program, and I think they have support of the dean of the medical school who now has an extra $200 million (thanks to Mr. Geffen) to work with. The med school is great, and will be modifying and updating their curriculum so it should be even better. And, the research is awsome, especially if you're interested in neurosciences or imaging (which I'm not, btw). But, the school is so big that there is something for everyone here.
I almost didn't come here for my interview, since I didn't know if I could stand to live in LA. But, that would have been a huge mistake. Looking back, I couldn't be happier with my decision.
Good luck all, and if you have any more questions feel free to email me.
Adam