1) Make the web version dependent on an internet connection. So maybe like you receive the next piece from the server side. This way if someone wants to play inside the stalls of a bathroom deeply fortified within a building, they'll need to purchase your iPhone version. I wouldn't advise using your iPhone on the toilet though, I read a pamphlet that says long periods sitting, especially with your asshole exposed, leads to hemorrhoids.
2) Make the web version dependent on a mouse, or a keyboard. For example, use the mouse dragging to determine where the block above is. As far as I know you can only simulate mouse clicks on the iPhone. The keyboard takes up most of the screen. Use these to your advantage.
3) Redo the web version in flash.
4) Put ads on the web version and give it away for free. See how this does.
5) If all else fails, block the iPhone safari user string.