I was recently asked to quote for building one - my first question was "why".
Surely a website that is built properly (i.e. to accessible web standards) should work / degrade gracefully and work on a mobile phone.
If you build a website which, for example, uses Flash for it's whole navigation then yes - it won't work on a iPhone - then again if you don't have Flash or are using a screenreader it won't work either.
PS: Don't build an iPhone app because you can - know your user; do they need an iPhone app... and why not build for Android, Blackberry, Symbian, Windows, etc while you're at it?