We were prevented from checking our bags all the way through to SIN when we had an overnight (~14 hour) stay at SFO. It was a drag because we had two big suitcases and bags of scuba gear that had to be hauled to the SFO Marriott and back.

In any case, when you check in, you can ask that your bags be checked through to SFO only. I did this when I had to pick up my Chinese visa in Los Angeles. Since I had sent my passport to a processing agency in LA and didn't have it at check-in at DEN, United checked my bag DEN-LAX. When I got my passport from the courier at LAX, I got my suitcase from the carousel, walked it upstairs, and checked it LAX-SFO-PVG.

Before trying this maneuver, I consulted FlyerTalkers. Some said that not all ticketing agents are aware that this can be done. I went to DEN armed with some printed posts and some handwritten notes from my conversation with the 1K desk. They weren't necessary: The 1K TA rolled her eyes and said people do it all the time. There was no problem with checking the bag to the gateway city (LAX). If you have trouble with checking the bag, you should ask for a supervisor, or get a UA rep on the phone. But I really don't think it will be a problem.

Good luck!

Lori.