Not adding to the rudeness on here, I'd like to weigh in:
What ithos describes is in fact syncing. That is, syncing calendars across devices.
And, agreeing with Csound1, it it NOT syncing across calendar hosting platforms. So far, so easy.
Now why would you want to sync across hosting platforms, or why would I want to have two identical calenders (if on different platforms or the same)?
(Almost) everyone has a google account, so just use google calender.
I never had any trouble handling my google calender from iCal (Snow Leopard or Lion) or iPhone (native calender, CalenGoo app, or recently WeekCal.). After finding out about the infamous "Step 9", that is.
So, all group related calenders are on google and I think it's not just good enough, but very handy to be able to show or hide these single calenders in my apps and in google calender's web interface.
My private calender is on iCloud at the moment but I'm thinking about going back to google, just because this app named Cue (tells you what your day looks like in a very nice and handy fashion) doesn't read iCloud. And if you want to share your private one with non-iCloud users, go ahead and switch to google. There is no single advantage in using iClouds calender that I can think of.
My use of iCloud is down to contact and notes syncing. Notes because of the handy integration in Mail.app.
Or did I miss the point? Is there a real-life use to syncing two calenders hosted on different platforms? (Other than the one-way-street method that ithos described) Maybe there is, just as much as there's use for tools like ifttt.com.