The Guinness Book of World Records has proclaimed the LG Optimus 2X "the world's first dual-core
smartphone." Although there are plenty of dual-core smartphones now, that can never be taken away, and it makes the proclamation official. something LG has been trumpeting for some time.
The details of the announcement say that:
The first mobile phone to use a dual core processor is the LG Optimus 2X, which was first made available to the public nationwide in South Korea, on 25 January 2011.
That device recently reached the U.S. as the T-Mobile G2x.