Our group of 14 did this back in 2007, albeit from ORD not SFO. We landed at the international terminal and went through customs, without claiming our bags that had been checked through to ZIH (interline). It was rather amusing, once we cleared customs we literally turned around and walked out of the same door we had just entered and back into the international terminal. We had to clear security once we left the international terminal to enter into the domestic terminal. All in all the process took about 20 minutes from gate to gate.
Coming back was a different story, we landed at the furthest end of the domestic side and had to walk the entire length of the airport, check in at the UA counter, and then go through security again. We had a 2 hour layover and were the last 14 people the board the UA plane to ORD. Needles to say, give yourself plenty of time on your return trip and get ready to walk/run what will feel like miles!
Hope this helps!
-Ualisthebst