He is in talks to develop and direct the film, according to THR. The remake will be produced by Twentieth Century Fox and James Camerson's Lightstorm Entertainment. David Goyer wrote the current script; he and del Toro previously worked together on Blade 2, and if things work out, they will reunite to develop the project.
The 1966 original was a medical thriller that followed a team that was shrunken to microscopic size inside a submarine and injected into the bloodstream of an important scientist in order to save his life. Tapping into then-current fears, the scientist had defected from behind the Iron Curtain. The team only has 60 minutes before they will revert to their true size. Veteran Richard Fleischer directed, capably, a cast that included Stephen Boyd, Raquel Welch, Edmund O'Brien and Donald Pleasance.
There's no timetable yet for the project, but del Toro is available because his planned followup to Pacific Rim has been removed from the release schedule. This kind of material is definitely in his wheelhouse, so I'd love to see what he could do with it. Will del Toro make a Fantastic Voyage that you'll want to take?