Lightning has CalDAV support today, on par with the support in Sunbird (where it debuted in December of last year). We're pretty committed to CalDAV as a primary "rich server calendar" capability, and we should be tracking the future drafts of CalDAV Access and Scheduling closely. We've been active in the CalDAV development process, and have a very good relationship with the Hula team at Novell, in addition to other CalDAV implementers like Oracle and Isamet.

We also have remote-ICS calendar support (often called "webcal"), including mvl's work to back those calendars with high-speed local cache for an improved user experience.

GroupDAV could be interesting, but I'm concerned that the restrictions of the protocol, and its potential incompatibilities with "standard" WebDAV and ICS make it hard to do well if you want to have good interaction between GroupDAV and other calendars. I'm not a GroupDAV expert, though, and I know that Stelian Pop has been working on a GroupDAV provider, so that might Just Happen. We'd certainly like to integrate well with OpenGroupware and other such projects.

One area of the new calendar architecture that we put a lot of thought into was the pluggable provider model. This should make it possible for people to add new calendar types as extensions, with a pretty seamless integration. I'm sure there are issues to resolve there -- we know of some integration points that we want to expose from different providers already, and Dan will be doing that work soon -- but the model itself seems solid, and should allow for new protocols and calendar formats to be added with a minimum of pain.

If we ever end up with support for Exchange, or the Brutus CORBA protocol, it'll likely be through extensions written to that provider interface. (That the Brutus IDL files are under GPL makes adding support directly into Lightning somewhat challenging, but it could work fine as an extension.)