Lost Treasures of Infocom has just been released for iOS. Zork I comes with it for free, and you can buy packages of games or the complete collection. The complete collection is $10, and for another buck you can get the Invisiclues for all games ($1 for all, not $1 each). There are some games missing, although I don't think anyone will be surprised that Hitchhiker's Guide the the Galaxy isn't there. It's really well done, with in-game access to Invisiclues, and the feelies have all been digitized.

Alternatively, if you own the games, there's a version of Frotz available for iOS. I tried it out tonight and loaded Hitchhiker's Guide in it, and it worked fine. Kind of a barebones interface, but what do you expect from a text adventure.