Even though most of us would rather this 42 be a 9mm than a .380, I still think it will fly off shelves. It says Glock on the slide, so that's reason number 1. Reason number 2 is that I think this will appeal to women and any recoil-sensitive folks. If it is a locked breech design, it should have less felt recoil than a similar sized 9mm. My wife dislikes shooting even low-power .38 Specials from a Ruger SP-101. I know that I am better with a pocket semi-auto than with a pocket revolver, and I think she would be too, if it was low-recoil.
If the reviews of the 42 turn out to be good, and if it's a locked breech design with less felt-recoil than a typical pocket 9mm or blowback .380, I might consider picking one up for the wife to try. If it is eventually made in 9mm with the exact same exterior dimensions, I will most likely buy one of those for myself, as well.
I'll bet that Glock already has plans to release a 9mm version, and they're just testing the waters with this first.