Someone put this up on Facebook, and you can see the whole letter here. Facebook also tells me that more than 15 percent of Chabad agree that the Rebbe is still alive, and perhaps hiding in a cave somewhere. (@Motel what say you?)
For me, the interesting thing is not that some people believe this nonsense (hey, don't all of us believe something nonsensical?) but that this particular nonsense seems to be such a close copy to something we've already seen.
According to one historical perspective, at least, Jesus, like the rebbe, was a highly charismatic preacher who collected followers by the score, followers who were emotionally attached to him and took him for a miracle worker. When Jesus died, there were some from his flock who were willing to let him go, but others felt the loss too deeply . They created circles of mourning, where they reviewed his teachings, told over the story of his life, and tried to make sense of his passing. Over time, these circles produced the authorized propaganda versions of Jesus's Life and Times viz. the four canonical gospels of Matthew, Mark, Luke, and John.
Where is Chabad in this process? Hard to say, but if there are messages going around between the faithful like this one calling on believers to spread the Good News about the rebbe still being alive, I'd say we're close to an official Parting of the Ways.
Search for more information about the rebbe heresy at4torah.com