I have a user connected via iCal to an overworked old CalDAV server (running OS X Server 10.5.8) who was doing fine in OS X 10.6.8. The data (from a number of groups, and his own individual calendar) would sometimes take a while to refresh, but it always came down eventually.
Once he upgraded to 10.8.1, his calendars would often have exclamation points next to them saying they couldn't connect. Manually refreshing would add "Updating..." to the title of the calendar window, but most of the time, the exclamation points would come back. I did some testing with LittleSnitch, and found that, if Calendar didn't hear back from the server for 30 seconds, it would time out.
Using the same CalDAV account on another machine with iCal in 10.6.8, connecting to the same server, it goes past 30 seconds and keeps going. (Maybe the default setting is 60 seconds; I haven't seen it triggered.)
Does anyone know if there's a way to tell Calendar in 10.8.1 to wait a little longer before timing out?