Mormonism is full of wacky mythology. Not any more than any other religion, but the Mormons do seem to have some peculiarly vivid bits of their backstory. It can lead to odd behavior in the here and now, too. None more so than posthumous baptism (or, to steal a term used in the discussion at PZ's blog, "necrodunking").
That's right. You may be a "Catholic the moment Dad came," but you can be a Mormon long after you've achieved room temperature. In a way, it's a very loving gesture. Mormons believe that baptism is a requirement for salvation, so they'll do it by proxy for those who never got around to it when they were alive. It's even better than the Christian deathbed conversion - you don't have to convert/repent at all!
It is also profoundly offensive to some people, for good reason. If someone rejected the Mormon brand of woo during his lifetime, either explicitly or by choosing some other brand of woo, baptism him posthumously seems a little sleazy. It's not as if he could consent, after all. So it's not surprising that a group of Jewish leaders objected to Mormon ex post facto baptism of, among others, Holocaust victims .
In that spirit, I'll object to the to the posthumous baptism of a pair of my "people," as it were. According to Famous Dead Mormons (via), both Carl Sagan and George Carlin, have had the deed done to them. Multiple times, in fact.* I'd imagine that would be offensive to both of them, being outspoken atheists while they were alive. What the fuck - they got Frank Zappa, too! No mean feat, given that the man declared cigarettes to be a food group while he was alive. Keep your grubby mystical paws off my idols, will you?
Or maybe it's not really worth getting worked up about. In the end, of course, it's all a bunch of pointless hand waiving. Posthumous baptism, just like the regular sort, is a ritual that derives meaning only for those who believe in that particular strain of woo. It has no more effect than a curse or a love spell. So, in the end, no harm no foul. If you get your kicks to play your little games in complete disregard of reality and the memories of the people you're recruiting, have at it. It's better than molesting the living, at least.
*Indeed, from the details on that site, most famous dead folks get necrodunked more than once. Whether that means they get multiple planets to lord over, I'm not sure.