I feel like turning that book into a movie would be like turning, I dunno, the computer game Myst into a movie. Half of the fun of Griffin and Sabine is the active reader involvement.
Or, anyway, that's how I remember feeling when I read it one zillion years ago.
Anyway, the relevant article is here:
Los Angeles based independent production house, Renegade Films, announced today that they have acquired the rights to the epistolary novels, Griffin & Sabine: An Extraordinary Correspondence. The 1991, best-selling series, written and illustrated by Nick Bantock, will be adapted into a feature film that travels through the three novels: Griffin & Sabine, Sabine’s Notebook and The Golden Mean. It is the first time the Griffin & Sabine trilogy has been optioned for film.
Bantock is in favor of the project, and judging from this old interview, it sounds like he'd have had to be super-confident in Renegade to let the rights out of his hands.
So we'll see.