tulip - they are a relatively new concept. They were originally made popular by destination weddings where the entire wedding (party & guests) would be flying somewhere vacation-y.

We got married in 2003 and even then, many, many people hadn't heard of them. (I got more calls from people asking if that was the invite or in an invite would follow even though the included letter said "invite to follow" ) Oh, and to be clear, we did not send S-T-D's (is that better?) to all our guests, just to those that had to travel by air.