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.
To use clients with iOS v5, you need at least DAViCal v0.9.9.7. It has been reported (cannot find the link, but I can confirm this) that the path to the collection should omit the collection name and the trailing slash, thus making the path end in <server>/davical/caldav.php/<username> (provided the server is not set up to serve clean URL's)--Vincent 08:47, 18 November 2011 (UTC).