Don't expect this for GL 1.3.x. The calendar code is currently the worst (no offense to those who wrote it) in Geeklog. I'm starting to work on the outline for a calendar plugin for GL2. My preliminary design right now would fully support the iCalendar standard which would mean you could share your calendar across many applications (including sharing on the web, with Outlook, etc). I'm just waiting on the Plugin API spec (hint, hint guys) to really get started.
The problem with getting this to work with the current calendar implementation is that events are stored in somewhat un-usefull way. It would be possible to write a php script which could take events directly from the database and export to the iCalendar format which could then be used with Outlook or other calendar programs that supports iCalendar. However, I'm looking forward to GL2, and I don't really need this feature so I have no incentive to work on this for GL 1.3.x.
You could probably write the script described above yourself as it shouldn't require any extraordinary programming, but if you're really desperate for this I'd willing to do it for some sort of compensation.