Here are few resources that will be very helpful for you as you design the look and feel of your iPhone app:

• The iPhone shell
• Screen size trend
• jQTouch - jQuery plugin for development
• PSD file of interface elements
• Mobileroadie : build your own app in minutes with popular features(video, audio, pay, social, etc.)

In case you need to add a CSS definition only for FF 1 or 2, check out this article (with examples): CSS hack for Firefox only