Because the CEDICT dictionary is available as a free resource, there are numerous Chinese-English dictionary apps on the iTunes App Store. Most of these apps are roughly the same and range in price from free to about 4 dollars. A few of these apps will include extra features such as bookmarking and simple flash cards. One of the best that I have found is the DianHua Dictionary.
The best thing about this dictionary is that it's free. As mentioned, it's based on the CEDICT project and contains tens of thousands of translations. This is pretty good for basic learning. The dictionary will accept english, pinyin, and drawn characters using the iPhone's chinese character drawing input (a feature that by itself can make the iPhone a worthwhile purchase).
The DinaHua Dictionary contains some handy features. First, it will save a history of your word searches so you can go back later to something you looked up – very nice when you forget. You can also bookmark definitions for repeated use. Bookmarks can be organized by named folders so you can have different categories. And finally, your bookmark folders can be used to automatically generate simple flashcards to help in your studies.
The DianHua Dictionary website plays a supporting role for the application. By registering with the website, you can back up your history and bookmarks. If your phone is ever lost or damaged (Zeus forbid!) your data will remain intact. Data can also be exported from the site in CSV format, I believe.
While the app is free, I highly recommend investing in the one essential companion app: DH Audio. This app provides an audio sample for many of the words in the DianHua dictionary. While it is limited, the voice quality is very good – much better than I have experienced with some other iPhone apps. Last time I checked this addition was only 4 dollars.