Disney does not offer these for sale to the general public. They do offer them to large groups and to convention/meeting attendees. I know that they've been around at least 2 years because I bought them in July of 2002 for a convention that I was at.
2 day/2 park and 3 day/3 park tickets have been available to the general public for many years. These are the non-park hopping tickets that are good for a single park on each day.
GoldenOldie is correct, the 2 and 3 day (one park per day) tickets have been available for years. They used to be called (and some people still refer to them as) bounce back tickets and are only sold at Disney itself. You can't purchase them in advance online.
There ARE 2 and 3 day PARK HOPPER tickets available, but as mad4 mentions, they're primarily released for conventions and large groups. There are ways to get them though, and one of the places is http://shop.gaydays.com/tickets/themepark.php . These are for sale by the org that runs Gay Days at Disney in June, but anyone can buy them and you can use them through December.