There's a bug in libkcal up to (including) the current version 3.5.7 that breaks the export of free/busy information. A patch exists for over one and a half years (see http://bugs.kde.org/show_bug.cgi?id=124495 also for a more detailed bug description), but upstream hasn't fixed it yet. I ran into the bug today and it would be great if the patch could be included in the kdepim patchset. It's no big deal, simply take the small patches from #5 and #8. It compiles cleanly for me and fixes the bug. If you need more information, help or anything, just let me know. Thanks.
Fixed in CVS in kdepim-3.5.7-r5 and libkcal-3.5.7-r2.
