An iPhone (or any other smart phone / tablet) is a mobile computing device. Applications that are designed for a mobile computing device should take advantage of the mobile nature of the device. Too many educational (cr)apps are designed simply as better flash card systems. They rarely take advantage of the most important affordances of the mobile devices they are on, and are easily replicated without using the technology.
Not only can you take pictures and videos of the world wherever you are able to travel, with a little bit of hardware, you can turn your iPhone into a mobile microscope, allowing you to view the microscopic world, which combines the mobile nature of the smart phone with its computing power. Your GPS in the iPhone allows you to participate in Geocaching.
Have an idea on the go? Use your smart phone to record a note about the idea, or a create a podcast on the fly. You can use the Internet capability of your smart phone to collaboratively keep track of data (or anecdotal observations) when out in the field. Heading out bird watching? Keep track of your GPS, a photo of the bird, and any other anecdotal evidence you need with that one device in your pocket.
The point is, try and find the educational uses of Smart phones which actually take full advantage of the capabilities of the phone, rather than limiting kids to using the phone as an extremely small computer screen.