A useful header tag for single-purpose webapps is apple-mobile-web-app-capable. When the user creates a home screen shortcut for the site, it will launch in 'fullscreen' mode, separate from the normal Mobile Safari application and without the URL bar or other chrome. If the site is nicely designed it can feel almost like a native iPhone app.
<meta name="viewport" content="width=320, initial-scale=2.3, user-scalable=no">
Allows you to set the width, height and scale values
<meta name="apple-mobile-web-app-status-bar-style" content="black" />
Set the status bar style, pretty self explanatory.
<meta name="apple-mobile-web-app-capable" content="yes" />
As Marc mentioned above, and is explained in the daringfireball.net link, allows the webpage to be run in full-screen mode, as opposed to through safari.
There's other various attributes that are supported and are documented here: https://developer.apple.com/webapps/docs/.../SafariHTMLRef/SafariHTMLRef.pdf
(need Apple Developer Connection access to view)
It turns out, there are a lot of them!
I found this one interesting:
To specify an icon for the entire website (every page on the website), place an icon file in PNG format in the root document folder called apple-touch-icon.png or apple-touch-icon-precomposed.png. If you use apple-touch-icon-precomposed.png as the filename, Safari on iPhone won’t add any effects to the icon.
precomposed is available to iPhone OS 2.0 and later
The DaringFireball link Marc shared links to the Safari Web Content guide. As mentioned by Andy, you have to sign up for it, but it's free and easy (well, not as easy as OpenID, but close).
The above mentioned documentation has moved. These are the new locations.
Safari HTML Reference: http://developer.apple.com/safari/library/documentation/AppleApplications/Reference/SafariHTMLRef/SafariHTMLRef.pdf
Safari Web Content Guide: http://developer.apple.com/safari/library/documentation/AppleApplications/Reference/SafariWebContent/SafariWebContent.pdf