To umm clarify what Devon_Hart wrote...actually, there are numerous references to Mentats' "Sapho-stained lips" in the text. The first reference I found (Dune, Berkley Paperback, p.98) was regarding Hawat, and of course in the Appendix. I'm pretty sure Piter's Sapho use or stained lips are mentioned somewhere in the text, but I can't find it just now. Perhaps Piter used less Sapho because his boss, the Baron Harkonnen, could afford more of the spice, Melange, for his favorite Mentat. Just about everything goes better with spice.
If indeed the chant appears nowhere in the Dune series, it should. It perfectly captures the spirit of Herbert's writing, and the Zen-like discipline of a Mentat.
In the movie, part of me would have preferred to have seen the Mentat Hawat recite the chant, as he was more old school than Piter, and seemed to be more of a balanced person who was still aware of his limits and the mixed blessing of the drug. It would have set him in contrast to Piter, the Twisted Mentat, who I see as an addictive personality intent on bringing down as many others as possible in the course of his own destruction -- he was already hopelessly addicted to spice, sadism, and power, and he knew his demise was nigh as soon as the Baron found him no longer useful -- what's one more poison to a man such as this?
Still, it could be argued that Piter had long since stopped thinking about the meaning of the chant and repeated it only as part of the Sapho ritual he'd been taught as part of his training.
I know, its a lot to read into a short scene in the movie, but as Devon_Hart and others point out, "it burns in your memory."