It's not the exact same effect, but I tried something and it worked out quite well.
Use THIS tutorial for the effect:http://www.gimpusers.com/tutorials/maki ... om-a-photo
But before you apply the effect, duplicate your start layer (Layer > Duplicate layer) and work on the top one, the copy. When you're done with the effect, make a selection around the person you want to have the effect on (with the lasso tool, paths tool etc, it depends on how you want it to look in the end), and then add transparency to the top layer (rightclick in the layers dialog > Add Alpha Channel), invert your selection (Select > Invert) and press delete on your keyboard.
Here's what I got in 2 minutes with that effect. You can do a lot better of course
Image source: http://www.sxc.hu/browse.phtml?f=view&id=1211151