Novell have released Evolution 2.6 for Mac OS X (2006-08-12). There’s a more recent reference (2006-08-28) to an unsupported release with the same version number. I’m downloading both, I wonder whether there’s any difference.
I’m surprised that it has taken me so long to learn of this product! Maybe the information washed over my head. It’s certainly true that when I last experimented with Evolution (possibly in version 5 of Ubuntu) I was disappointed in support for iCalendar publish and subscribe.
Now, I realise that Evolution supports CalDAV.
There’s also an unofficial port of Evolution 2.6.5 for Windows.