NOTE: Some of the software is very experimental!!! I had some issues that my .ics was totally removed when I added a new 'task' via lightning, please make a backup of your ics file!
Some while ago I wrote a small article about WebDAV and Calendar files. Until so far no results on reading the files into my Nokia , but... I found an extension for thunderbird to handle meeting-requests from Outlook clients
You can find information on the project website of 'lightning'. To configure your webdav, you need to click on the calendar button
Click in the left pane with the right button en select 'New Calendar', now you can use the wizard to select your calendar.
And now let the invitations come...