I've seen lots of pictures and videos of trees planted that way with good results. Right now most of my trees are planted five foot apart. However I'm planning on planting four fig trees per 30"X30" raised bed this season which would be like the first example in the link you provided.

There is a member in the Fruit & Orchard Forum who has hundreds of trees planted this way with good results. Keep in mind this type of planting requires a lot of maintenance pruning as the trees are supposed to be kept fairly short.

These are the two best examples I have found on these type of plantings.

How To Thin A Fruit Tree
http://www.youtube.com/watch?v=u7oyVW6zwVY

Here is a link that might be useful: High Density Fruit Tree Growing

I checked out the links. While I like the idea, it's going to be really hard to chop off my new trees to knee height!
I'm thinking about trying the 3 trees in one hole option with nectarines, apples, peaches and plums.
One thing that I wonder about is that they use a summer pruning method in their pruning programs and usually around here, you don't want to prune things drastically in the summer.
I can see where you do have to keep on top of the pruning though and I am telling myself that it will be a lot easier and less painful than trimming my 200 rose bushes :)

thanks for sharing your find agility_mom .... ;)