It's now available on an authorized R1 DVD on a manufactured-on-demand basis from Sony Pictures
http://www.sonypictures.com/homevideo/columbiaclassics/the-films/film.php?catalogid=19341&sphesku_index=0&upc=043396354913
I have it, it's a great widescreen transfer. No extras but the trailer, though.
It's also available on Region 1 DVD from Mill Creek Entertainment as part of a box set called Vintage Sci-Fi 6 Movie Collection.
I am the Duke of IMDb bio writers! I am A#1!
All six films in the Mill Creek set were previously released as singles on the Sony Choice MOD line and are also still available, including The 27th Day.
shareI've just received the Millcreek set and watched THE 27th DAY. The film is quite good and the picture/sound quality are excellent. For the price, I recommend this set over the individual Sony MOD releases.