Vice President Biden said in an interview that it would be "ill advised" for Israel to attack Iran. He also said that Benjamin Netanyahu (the Israeli Prime Minister) was not going to strike Iran, but issued the statement anyway (http://www.latimes.com/news/nationworld/world/la-fg-biden-israel-iran8-2009apr08,0,2388127.story). It is unclear why Biden does not think Netanyahu would bomb Iran. After all, Netanyahu told us that if we did not stop Iran they would (http://www.theatlantic.com/doc/200903u/netanyahu). Israel has also thrown out a few warning shots as well. The strike in Sudan (Berman Post: Israel Launched Air Strikes Against Sudan) showed that they could reach out and touch Iran. The successful missile defense test (http://www.haaretz.com/hasen/spages/1077217.html) showed that Israel was capable of defending itself against a potential Iranian counter strike.
I think that Israel believes it would be even more ill advised to wait for a mushroom cloud to appear over Tel Aviv before taking action, even if Biden does not think there is a real danger.