Native CalDAV support in iPhone was introduced with software relase 3.0 in july 2009. DAViCal 0.9.7 was released shortly afterwards to support this. OS release 3.1 fixed some bugs and introduced some new ones, and DAViCal release 0.9.7.2 is needed to work with this.
Instructions for configuring the iPhone to use a DAViCal server are on the client configuration pages, here.
With version 0.9.7.2 it is also possible to make some specific server configuration adjustments which act to simplify the configuration experience for Apple (including iPhone) users. Full details of this configuration is discussed under Setup for Apple Users.