The potential for this application is enormous. Many report using it with far more success than normal hearing aids in noisy environments. BUT: You are currently restricted to wired ear phones/headsets, physically tying you to the phone*. The ideal would be an inconspicuous ear mounted bluetooth earphone with no microphone.
*After much research I found a single exception: from Creative Labs - their Bluetooth enabled WP-300 over the head/over the ear headphones. The hundreds of other wireless headsets include a microphone designed to capture the wearers voice. These headsets automatically disable the iphone mic or a mic plugged into the phone.