Health insurance is state by state--I can't keep my WA health insurance plan if I move to CA, for instance. Since YC already says "settle in Silicon Valley", I'm sure it would actually be easier for them to buy a deal from a CA health insurance company, though the legal wrangling would undoubtedly be epic.

pg has remarked multiple times about why he doesn't provide office space, though. Not sure if the same rationale applies to health insurance.