If President Bush decides that the use of nuclear weapons in Iran is warranted, I can't imagine that he'd subcontract it out to Israel, because:
1) If we sanction a nuclear attack by Israel, we'll be on the hook to defend them against whatever comes their way as a result of it, and Israel (unlike the United States) is within the direct military reach of the entire Arab world.
2) If Israel drops nuclear bombs on Iran with our blessing, that will necessarily reveal theit possession of nuclear weapons in violation of God knows how many international laws, *and* that we knew they had such weapons. If the radioactive material turns out to have our signatures, that will prove that we *supplied* nuclear weapons to Israel. . legally and diplomatically, we'd be better off just dropping the bombs ourselves.
3) If Bush leaves America holding the smoking gun, the next President will be committed to dealing with the consequences of the bombing as a matter of national survival. If Israel does it, the next President will be left with the option of cutting his losses if the war goes badly.
4) If Israel hits Iran with nuclear weapons, there's a very strong likelihood that they'll face nuclear retaliation from China or Pakistan, among others. If we do it, nations sympathetic to Iran will be forced to weigh the much scarier prospect of starting a nuclear war with the United States. At the end of the day, the world will settle for dealing with us by diplomatic isolation and economic sanctions. (Probably. . . )
The only way I see Israel carrying out the attacks themselves is if we tell them not to do it and they slip the leash. I honestly don't know what would happen in that case. Bush would probably commit to fully support them anyway; Obama would commit to defend them as well, but he might be tempted under these circumstances, to seize the political opportunity to drive a very, very hard bargain with them. (e.g.: by dragging the Israeli Prime Minister to a bargaining table and simply slapping a peace treaty down on the table in front of him that follows the lines of previous bargaining positions but resolves all the outstanding issues left at the last round of negotiations in the Palestineans' favor. If Israel signs it and the Palestineans don't, we'll commit to defend the current borders as they stand. If Palestine signs it and the Israelis don't, we'll commit to defend them for 15 years and offer visas, travel vouchers, and permanent resident status in the U.S. to any Israelis who want to emigrate here. Whether they decide to stay there, move here, or move somewhere else, after the 15 years are up, we're pulling out and ceding the entire country to Palestine).