I thought the etiquette surrounding this question would be sort of a no-brainer, but since I've seen a lot of message board confusion over the issue, I thought I'd go ahead and address it here.

Save the date (STD) cards are an optional way to spend money notify your guests of your wedding date before sending out invitations. You are not required to send them out and if you do, you are not required to send them to everyone on your guest list. You can save money by sending them only to out of town guests and people who will have to make arrangements well in advance, with more time than the invitation allows.

If you send them and to whom is up to you, but there is one very important rule of etiquette that you must not break. I'm really not even sure why this is a question at all, but here it is: Do not send a save-the-date card to someone unless you are definitely inviting them to the wedding! You are telling them to set aside this date, so if you end up not inviting them, you see how this would be a huge slap in the face. Those who get save-the-dates expect invitations as well, and rightfully so. As far as save-the-dates are concerned, this is the only steadfast rule of etiquette. Those who break it will be immediately labeled with bride- and groomzilla branding irons.