I think the iPhone is reïnforcing for me is that web browsers still aren’t a good universal client, despite how far they’ve come and despite their success in that area. I almost always prefer a stand-alone iPhone app to accessing the site itself via Mobile Safari. Even sites that are optimized for Mobile Safari. Heck, I sometimes prefer the iPhone app to the site on my desktop web browser.
While I haven’t tried my hand at writing an iPhone app, I suspect they’re often easier to write than the web app itself.
There’s also the observation that the optimized-for-Mobile-Safari version of a site is often better than the site itself. So how much of it is that web browsers make lousy universal clients and how much is it that they merely aren’t well used?