The iPhone App I used in my recent March 2010 trip was WDW Wait Times. It is a free App (though they do have a similar app that is not free that removes the ads). This app asked users to input wait times so it is only as good as the people using it.

Another App I used was WDW Park Hours. It listed the park hours everday (and the Extra Magic Hours). This was very useful app especially if you have a chage in plans. This app was also free (but had a not free cousin).