Mormons believe proxy baptisms give those in the afterlife the option of joining the religion. It's primarily intended to offer salvation to the ancestors of Mormons, but many others are included.
Baptisms for the dead are performed inside Mormon temples, with a church member immersed in water in place of the deceased person. Names of the deceased are gathered by church members from genealogy records as well as death and governmental documents from around the world.
The article also mentions that Ghengis Khan, Joan of Arc, Adolf Hitler, Josef Stalin, Buddha, and Anne Frank have all been baptized in this way. No offense to any Mormon readers, but this is pretty loopy. Isn't the point of religion to determine your status in the afterlife? I mean, why not just be a hedonistic pagan and ask to be baptized by Mormons in your last will and testament? It's even more of a get-out-of-jail-free concept that the whole deathbed-conversion thing.
And I don't really understand why its offensive. Is there a religious or doctrinal conflict here? It seems about as pointless (and ridicule-worthy) as wrapping the bodies of dead muslims in pigskin. They can try to baptize the Prophet SAW too if they want, but it's their time to waste.