Well, it's just one of the many. There's PhoneGap for example, and it's free.
Just remember in both cases you need a developer certificate in order to get stuff into App store, and that'll cost you a bit.
For adventure games, a cheaper (easier, quicker, more dark side...) way of deployment would be to release them as web apps with offline capability. The only thing which currently sucks with those is the sound support.
Hopefully GM will make html5 output to his engine some time soon. That would make drag and drop possible on a browser, for example. I haven't seen a game engine optimized for making adventure games outputting html5 - yet. It's bound to happen sooner or later.