True, and they don't even have to get that close. A large passenger jet on takeoff leaves behind it a HUGE heat signature and, because it is moving relatively slowly at a low altitude, is an easy target for a MANPAD.
However, before you all go cancelling your plane tickets, remember that the missile will home in on the engine, NOT the passenger cabin. Almost all airliners today are designed to be able to fly and land safely even with on engine completely missing from the wing.
It would make spectacular news footage, but the likelyhood of a MANPAD actuall bringing a 747 DOWN is pretty slim.