While checking up on the Darwin Calendar Server wiki the other day I noticed something I had missed last week: CalDAVTester. It is an exhaustive suite of tests written in Python with XML config files to verify that a CalDAV server implementation is properly implementing the spec. This suite of tests is going to prove very useful as more servers and clients implement the CalDAV spec.
Right now the biggest problem with CalDAV is a lack of clients and servers. That will change over the next 6-8 months as clients and servers are refined, released and rolled out. Hopefully the CalConnect group and an exhaustive suite of tests will help keep interop a high priority.