I don't know any ways to share iCal calendars in such a way that both can add items to the same calendar (it might be possible with some creative use of aliases). But, you can always use the "attendees" field to invite the other to join "events" or you can publish or export some calendars.
Note that you don't need a WebDAV enabled web server to publish your calendars, just make ics files and link to them this way: webcal://www.geek.no/_ical/Ukenumre.ics
As for photos in iPhoto, the best way might be to turn on sharing and enable fast user switching like the iTunes tip... but I've never actually tried to see if this will work...