I've been working on these a lot, porting old sites to iPad/iPhone compatible ones. I think they're a ton of fun. I mostly use them as tools around the house and being out and about. iOS 4.2 just opened up a lot more HTML5 options, so I've been experimenting a ton with WebSockets lately.
I've been working on this guide as a how-to for others. You can really make some interesting tools and if you spend a good amount of time on them, they can run just like native apps. I've always wanted to make iOS apps, but the whole SDK and Mac-only dev platform were huge turn offs.
Anyways, here's my blog link. This is just a little hobby of mine, would love to see who else has any fun making these sort of sites or even if others have suggestions.
A Definitive Guide to iOS Web Apps | ryannystrom:\blog