Specifically:
1. Is distribution a much bigger challenge. Does the new iphone web app directory on apple help.
2. Assuming that there is no equivalent website so that the app has to develop traffic from scratch.
Here's my webapp in the directory.. http://www.apple.com/webapps/sports/ridertech.html
After falling off page 1 in the directory, it'll provide very little traffic so you basically need to promote the iPhone-specific site via your other web/mobile/social apps.
If you have no existing user base or app traffic to draw from, you may want to build a simple mobile-xhtml site first because there are a lot more non-iPhone users. It also helps remove some of the design challenges and focuses your efforts on the core content paths.
Feel free to contact me directly if you want more help - geohelper [at] gmail [dot] com