iCloud seems to be open to non-Apple clients now. Let's test and enhance that.
Did anybody manage to configure icloud?
I just gave it a try in reaction to your question.
Discovery via https://[www].icloud.com fails for various reasons.
Using one of the urls mentioned elsewhere (google "icloud caldav url") leads to a different error:
[<?xml version='1.0' encoding='UTF-8'?><error xmlns='DAV:'><Require-User-Agent xmlns='http://cal.me.com/_namespace/'/><error-description xmlns='urn:mobileme:davservices'>User-Agent Header is required</error-description></error>]
Adding a User-Agent is possible, but will require some changes to the source code. Let me know if you can and want to work on the source and I'll provide pointers.
Contacts are said to work with 1.5.
Calendars need support for downloading changes via sync reports (https://tools.ietf.org/html/rfc6578) because the current approach is not supported by the iCloud server (Apple issue 17001498 "CalDAV REPORT drops calendar data").