The water-filled sleeping sac that Heinlein imagined preceded the waterbed by several years. In fact, the person who invented the waterbed was unable to obtain a patent for it because its appearance in this novel constituted "prior art".
There is a real-world religious group which is based upon the trappings of Mike's teachings. It's even rumored that Heinlein wrote the novel on a bet with L. Ron Hubbard (founder of Scientology), to see which one could start a bigger cult, but decided to call the whole thing off when it started getting a little too real. That story, however, has never been confirmed.