If you're a web developer with a hankerin' to write for the iPhone, this is for you. Apple has posted an informative webpage that the outlines iPhone-specific guidelines you'll want to keep in mind, as we as some more general best practices. Topics include:
Integrating Mail, Safari and Maps
Understanding what Safari supports in iPhone
Optimizing page readability
There's more, of course, and you can get the full run-down here. Now get to it!