Sophisticated digital hearing aids with nice filters are very expensive. The digital filters can be adjusted based on your hearing test to amplify the sounds you have the most trouble hearing. Most hearing aids require you to pay a visit to your audiologist for any sort of adjustment, except for these. The iPhone has a rediculous amount of computing power, a microphone, and headphones. Using the iPhone as a hearing aid device is not a novel idea because a few apps exist already: iHear Hearing Aid, soundAMP, Listen Up, and amplitude. However, these applications seem to use no filtering or very simple highpass or lowpass type filtering. These apps do not allow for any sort of multi-point or continuous filter definition based on the results of a hearing test.
My suggestion is to use FFT Optimal (Wiener) filtering entirely in the frequency domain. This allows a continuous filter curve to be applied to the audio data instead of a simple bass or treble cutoff filter. I implemented FFT filtering in my JHSV Analysis Engine work, but I used a single cutoff frequency for lowpass filtering instead of a continuous filter curve. The algorithm is the same except instead of zeroing the frequency-domain data with a cutoff frequency, the results of a hearing test can be multiplied against the frequency-domain data for the equivalent of tens or hundreds of adjustment channels. Some sort of interpolation can be used to make the filter curve continuous between discrete points. A simple highpass or lowpass filter is far too coarse for natural sound.
I also suggest adding a button to profile the background noise of a situation. Consider a group of friends sitting in a noisy restaurant. A hearing-impared person could wait for a lull in the conversation and record several seconds of background noise to create a frequency curve (FFT) of the background noise. This curve could be used alone to try to remove the background noise during future conversation (more FFT filtering) or it could be applied in addition to a hearing test frequency curve for additional amplification of selected frequencies (with FFT filtering). This will have the effect of increasing the signal to noise ratio of friends' voices. It should work like Bose noice-cancelling headphones except the noise profile is pre-recorded and not real-time.
Either way, the key to more natural sound and a more helpful hearing aid is frequency-domain filtering and continuous filter curves.