Setting up a WebDAV server to sync to OmniFocus on your iPhone will put a copy of your OmniFocus database on the WebDAV server.
The file format is a series of XML files compressed into .zip files. There is no public documentation of the format. So it's somewhat human-readable, but not as convenient as plain text or something.
For example, I just added a task to my OmniFocus database, and the XML for it looks like this:
Code:
<?xml version="1.0" encoding="utf-8" standalone="no"?>
<omnifocus xmlns="http://www.omnigroup.com/namespace/OmniFocus/v1" app-id="com.omnigroup.OmniFocus" app-version="77.9.0.103820" os-name="NSMACHOperatingSystem" os-version="10.5.4" machine-model="MacPro3,1">
<context id="pH_AComJRtO" op="reference">
<added order="4">2008-07-06T00:37:48.553Z</added>
<modified>2008-08-05T06:05:16.837Z</modified>
<name>Computer</name>
<rank>-2</rank>
</context>
<folder id="pfTns_620w9" op="reference">
<added>2008-07-06T04:20:05.864Z</added>
<modified>2008-08-04T23:17:35.923Z</modified>
<name>Work</name>
<rank>-1610612736</rank>
</folder>
<task id="mqK_FgC6HiH" op="update">
<project>
<folder idref="pfTns_620w9"/>
<last-review>2008-07-25T07:00:00.000Z</last-review>
<review-interval>@1w</review-interval>
</project>
<added>2008-07-26T01:01:08.009Z</added>
<modified>2008-08-07T01:27:29.740Z</modified>
<name>Testing</name>
<rank>2147475456</rank>
<order>parallel</order>
</task>
<task id="flXIcU_LX47" op="update">
<task idref="mqK_FgC6HiH"/>
<added>2008-08-07T01:27:19.534Z</added>
<modified>2008-08-07T01:27:29.740Z</modified>
<name>demo file format</name>
<rank>1610612736</rank>
<context idref="pH_AComJRtO"/>
<order>parallel</order>
</task>
</omnifocus>