I have the iTOUCH and think the dining app, as well as the TripIt app both seem great. The TripIt app would come in handy because it lists your daily itinerary---meaning you can store what park you're going to, confirmation nos for ADRs, etc. in it.
That being said, I think the iPHONE will be much more handy than the iTOUCH in the parks, which is one of the reasons I'm happy I upgraded recently. Some of the apps are only accessible when you are connected to the internet via WiFi (such as the best/worst days, show times, etc.) and as far as I know, the parks do not have free (or for pay) WiFi available. With the iPHONE you are constantly connected to the 3G network so while service might be a little spotty---you should be able to use all of the apps you have at some point and somewhere in the parks.