I am just wondering whether I can ask for the stamp upon enetring US at the SF airport. I have read in the forum that JFK is the only airport issue the stamp but I am not sure whether I can get the stamp at SFO if I ask.

You can ask, but they will probably not issue one or even know what you're talking about. If you do get one, please make sure to post and let everyone know, that would be some great information.

Can't hurt to ask. Be nice, nice, nice.