They might not have any choice if they truly believe that Iran is an imminent nuclear threat.
There is a very good chance that they will act now, before the end of the year, while the current US administration is still in place, because if Senator Obama is elected POTUS, then Israel may perceive that they will not receive the same degree of "backup/support" as they will certainly receive from the current Bush administration.
Nor would I be surprised if the US was involved in a joint operation with Israel.
What about the Arab reaction? Well, Iran is Persian, and Saudi Arabia is not too happy with the Iranian attempts of late to exercise influence in the region. Iran is acting too "Persian-like". So Saudi Arabia, and other Arab nations would turn a blind eye to military action against Iran.
Feb 2007. Report: 3 Gulf states agree to IAF overflights en route to Iran:
http://www.haaretz.com/hasen/spages/830309.html
-Astro