I should have mentioned: I was assuming the recipients weren't necessarily PGP users. Realistically, most people aren't. I'd imagine a dead man's switch would often be used to send documents to law enforcement, lawyers, journalists, etc. How many of those people have PGP public keys? If they don't, then you have to ask them to create one. Besides the difficulty in getting people to adopt PGP, you're also back the problem of disclosing your dead man's switch prior to your death.