I did an engagement announcement which had at the bottom "March 2012 Destination Wedding is Planned" this was in Oct 2010. I figured that would give a rough estimate on the date since we didn't have a resort or date picked yet - only the month/year. I plan on doing invites which will include both the Mexico wedding and an AHR invite (which is being held in a different state than where we live). I'm hoping to get this out by Sept 2011 with an RSVP time of Jan 2012 so we know the numbers both in Mexico and for the AHR.
Planning everything from far away is proving difficult!