When Israel militarily responded to Lebanese Hezbollah, many people wondered if World War 3 would start. People wondered about World War 3 with Afghanistan and Iraq as well.
I dont think its possible to have another World War. Nations and governments are too economically, politically, and socially linked to one another that the possibility of World War is negligible. World Wars are constructed by strong military alliances and deep-rooted global strife. In today's situation, the deep-rooted strife remains only within the Middle East, and mainly among Israel, Iranian/Lebanese Hezbollah, and Hamas. While President Ahmedinejad did say that an Israeli attack on Syria would bring Iran into the war (which would then bring in the US), the chain of threatening military alliances ends there.
Europe would mostly remain out of this war (though aiding US and Israel), Asia would stay out (though China may still consider trading with Iran for oil-energy benefits), and the rest of the world is mainly uninvolved.
So the final count in what is probably the worst-case scenario is: US + ISRAEL vs. IRAN + LEBENESE HEZBOLLA + SYRIA
which isn't even that bad.
What the world must not lose focus on is Iran's nuclear program. There have been "talks" and nothing has been done. The UN must ACT now, and show the world that it has any sort of power or sway in important world issues. if the UN/G8/EU/ (insert shortened form of an organization) dont act soon and efficiently, then the US will once again invade the Middle East to get something done.
World War 3... no.
Energy crisis if Iran decides to screw with the Persian gulf when US/ISRAEL attack it (because of nuclear program and general violent attitude toward Western society)... most likely.