I think you can charge more for iPhone development versus web development just because there are fewer folks with those skills. I've only done a few contract jobs but no one has every argued my rate ($100/hour). I always charge by the hour and I would suggest doing this, especially if they don't have a detailed spec. Most of jobs are with folks who don't have a background designing software (since now everyone thinks they can get rich off of a dumb iPhone app) and so its inevitable that the project will change. This way, if they want to tweak some feature, they have to pay.
You can also discuss taking a cut of the app revenue in exchange for a discount, but i would be wary of this. I am doing it for one project because the project was smaller in scale, so its less of a gamble. I had another that I almost offered a 50% discount in exchange for 50% equity... and that app has made about $10 in the month or so its been in the store. So, my experience tells me avoid royalty arrangements....
You can also discuss taking a cut of the app revenue in exchange for a discount, but i would be wary of this. I am doing it for one project because the project was smaller in scale, so its less of a gamble. I had another that I almost offered a 50% discount in exchange for 50% equity... and that app has made about $10 in the month or so its been in the store. So, my experience tells me avoid royalty arrangements....