I've non-revved to Shanghai and Bejing on United probably 8 or 10 times from SFO. I was never asked for any onward tickets and have never had any problems with customs or immigration. The Shanghai airport is really nice.

The only problem I had was one time a Beijing United CSR tried to snake me out of a first class seat, claiming that I hadn't listed for it. BS- she wanted the seat for a buddy.