Another important argument in favor of a native app over a webapp is the iPod Touch - sales of the Touch are doubling year over year, getting pretty close to the iPhone, in terms of units sold.
Requiring iPod Touch owners to connect to wifi before they access your webapp just puts another hurdle between you and your users, one which they won't always be able to clear.
That is a great point, although there are some things you can do with local caching it's not really a solution in that instance.
My main argument is, as a developer you KNOW what the App store approval process is like, it's been beaten to death all over the Internet. So either accept that you're going to have to play by Apple's rules or make a web app and save yourself the trouble. It's Apple's garden and we all just play in it.
Requiring iPod Touch owners to connect to wifi before they access your webapp just puts another hurdle between you and your users, one which they won't always be able to clear.