Well, there are now two ways to write applications for the iPhone.
You can basically either write official apps, or unofficial apps.
Official apps are written with apple's SDK, which they recently began providing, and once you make an app, it has to be approved by apple and they distribute it via the iTunes app store that opens July 11. You can easily charge for these apps but i'm not sure you can give them away for free, or how much it will cost you.
The unofficial way involves unlocking read/write access to the file system of your iphone or itouch so that you can install any applications you want, and then writing the application using existing tutorials on the iphone's file system. It is incredibly easy to unlock read/write access for your phone (a process called "Jailbreaking") using any number of freely available apps online (i used iLiberty+) and just a few clicks. Once you jailbreak your iphone, a program is added that lets the user install any 3rd party program they want, right from the iphone. If you are a 3rd party developer, you just make a simple XML file on your site that describes what software you have available to download, and an iPhone user can add the address of that file to the "sources" list in that program, and your software will instantly become available for them to download. You can still charge for software by including unlock codes in the software and giving people a paypal link, or you can give it away for free if you want.
I suggest going the "unofficial" route because it is the best way to give it away freely, which i would want for a project like this.
If you google for "iPhone programming" you'll find plenty of helpful resources on programming a jailbroken iphone. I found this link with that search:
iPhoneDevDocs
It looks decent.
But anyway, i was really hoping there'd be a stronger response. Maybe it's just because you guys haven't seen what a jailbroken iphone is capable of, but there are many hundreds of 3rd party apps written by people from all over the world to do any number of things, it is completely realistic for us to come up with a datalogger app for our cars that runs on this platform!
-Taylor Alexander