Hi!! I know there is so much drama with planning a wedding. I would say that a year in advance would be the earliest I would send out the Save the Dates. You don't have to put the destination if you don't want to, as long as you have the wedding date set. You can keep it simple by having the date of the wedding and mention something like STD for our destination wedding, Feb.14 2011, invitation to follow. You will confirm the actual location and resort on the invitation (this should be sent out around 6 months in advance, in my opinion).
The whole bit about having to have a group number to secure a venue I think is crazy. Yes, you should have an idea if you want to allow children at your wedding and whether you want to keep it big or small, but you should not have to come up with a number this soon in the game. I still dont have a number of guests coming and my wedding is in November. I say tell her whether you want a family or adults only resort, areas you are interested in and if you prefer the wedding on the beach or elsewhere just to narrow down resort choices and let her know that at this point you dont know how many guests are coming.
I booked my wedding a year in advance BTW. Your wedding date is very popular since its valentines day..I would book as early as possible.
Hope this helps!!