When I was interviewing at California schools I heard a lot of rumors, from other applicants, that the UCs did actually show some preference toward CA residence for their MSTPs. However, I think you should listen to Gfunk and disregard these rumors if you happen to hear them.
UC undergrads almost always want to stay in California, so there are a lot of CA residents that apply for the MD and MD/PhD programs. This can obviously lead to a disproportionate number of CA residents who end up going to the school. Combined with the fact that the MD programs do have a preference (sometimes requirement) for CA state residency, this is probably the source of such false rumors.
good luck!