There are are dozens of Interactive Fiction interpreters for just about every platform out there (Windows, Mac, Linux, Palm, etc) and now one of the best is available on the iPhone / iPad. In fact, I think iFrotz on the iPad could help make Interactive Fiction more popular than ever. What sets this application apart is that its got a library of a dozen built-in games and the ability to easily add more from IFDB or your own collection. You can also tap on any word in the text to add it to your command or double tap to get a command history.
The virtual keyboard works surprisingly well, especially on the iPad. And as usual, you can customize the colors and fonts to make it look like the old school monochrome green screen of your dreams. Best of all, you can take your games anywhere. Load up with Zork, Deadline or any of thousands of games available on IFDB and you're ready to roll!
The easiest way to get Frotz on your iPad or iPhone is to install it directly from iTunes.