I would definitely not lie about it as other people here have recommended. I would not burn a bridge with your current employer either. Here's my take on things: If the startup in SF wants you, and if they are WORTH working for, then they will accommodate a schedule change, recognizing it's happening very fast and you have other obligations. In fact, they should respect you for saying -- Hey, I don't really love my job, but I've made a commitment for Monday and don't want to leave them hanging. Can I fly out over the weekend or on Tuesday or Wednesday?

If they aren't willing to work with you on this, then you need to look for something else -- IMHO!

Never burn a bridge. Always be honest. It's a small world and it's not worth it. Not only does it affect the way other people think about you, it affects the way you think about yourself. Build character one step at a time, not the reverse.