i have been mulling this over myself recently.
with the war between lebanon and israel at its beginning, i can imagine that iran will be watching closely. iran if you remember suggested that israel should be wiped off of the planet and may well be tempted to help lebanon, the enemy of my enemy etc. now, israel has nuclear weapons, lebanon and iran do not. im sure that if their very existence was threatened israel would use its nuclear arsenal against iran or lebanon. this could well provoke further attacks against israel.
before this though, iran has been wanting to develop nuclear technologys for itself against the requests of the western world. if iran was to invade israel, our interests would be compromised as irans success would mean that iran would have these nuclear weapons which we do not wish them to have. this would give us every reason to enter the war on israels side. naturally, us helping the jews could compromise our relationship with egypt, who is a close ally, and other muslim countries, possibly causing them to take arms against us if it was percieved to be a muslim oriented attack.
could this be the start of WW3? i think it depends on iran. however, i have every confidence that israel could defeat lebanon and iran at the same time.