Start here:
Really general overview:
http://en.wikipedia.org/wiki/CalDAV

This is a good overview of the specifications:
http://caldav.calconnect.org/standards.html

Here are some of the standards we'll be implementing.  Note that several of these documents are referenced in our target spec:
http://www.ietf.org/rfc/rfc3253.txt
http://www.ietf.org/rfc/rfc3744.txt
http://www.ietf.org/rfc/rfc4791.txt
http://www.ietf.org/rfc/rfc4918.txt
http://tools.ietf.org/html/draft-desruisseaux-caldav-sched-03

Overview of the specification dependencies and documents:
http://www.rooftopsolutions.nl/article/227

Google's instructions on using Sunbird (free/open source calendar software that supports CalDAV as a client) with Google Calendar:
http://www.google.com/support/calendar/bin/answer.py?answer=99355

This is a document that outlines the coding standards that will be in effect on all software we write:
http://wiki.horde.org/Horde4/CodingStandards

Code resources for CalDAV and GroupDAV:
caldav:
http://rscds.sourceforge.net/
http://rscds.sourceforge.net/clients.php
http://wiki.zimbra.com/index.php?title=CalDAV_with_Leopard_iCal
http://www.rooftopsolutions.nl/article/232

groupdav:
http://www.egroupware.org/index.php?page_name=wiki&wikipage=GroupDAV
http://www.nabble.com/GroupDAV-and-IcalSrv-1.4.003-td15045378s3741.html

DavMail: (CalDav Gateway with Exchange)
http://davmail.sourceforge.net