Their are to many large and powerful groups within our media, congress, and the upper class for the US to remain Neutral in a Israeli-Iran war. The ultraconservative base in particular would drag the nation into the war on Israel's behalf. Plus Iran represents everything the west hates about the middle east, so it would be easier than people think to gain popular support for American involvement. Ultimately, Israel and the US are tied at the hip right now. You can't fight one without the other coming to help.
It would be Israel's war since it declared war first. If the US does get involved, then I might say that the Israeli government was using the powerful force of the US military to take down the Iranian government. I would be surprised if Americans would agree to this as they see what Israel did was right and that the US should always what the Israeli government wants it to.
If Israel declares war on Iran that is Israel's war. If Iran declared war on Israel (and not merely in reaction to Israel declaring war or invading Iran) then I would say we should get involved. But if Israel is going to go and do something as stupid as starting a war with Iran then Israel should be on their own.