Am I wrong in thinking that the same ICS (ical) file could be used across Google Calendar, Outlook and iCal to sync calendars from a central source? Having dug around I can see windows phone won't sync, just import and Android will have to sync with their Google calendar first but I can't see any compatibility issues.
Am I wrong?
We do exactly this.
Both Outlook and iCal are a bit picky about the precise format of the input; the most reliable method I've found is to subscribe to your source ICS file in Google Calendar, then make that Google calendar public and use the resulting Google ICAL link for the other clients. If you go this route, I recommend using a Google account that you don't use for anything else, as if you delete the Google calendar subscription for any reason the links you've shared will be null and void.