A selk'bag is an all purpose sleeping bag which has arms, legs and hands which you can open. essentially its all the fun of a sleeping bag with few of the drawbacks.
I have one and I have to say they are very, very comfortable and warm (down to -8 C), be sure to choose the right size though. im 181cm tall and the large was just too big for me; I had to pull it up everytime I wanted to walk somewhere which is annoying considering that was the whole reason I got it!!
Its very nice to wear and it doesnt matter if you have clothes on or not. The open hands are also a very good feature, allowing you to eat, work, play xbox ect all the zips have internal and external zips so you can open with from the inside. the one drawback i can see, from a male point of view is the fact there is no "fly" so you have to take it off should you want to just pop to the loo.
Overall a very good product with a few minor drawbacks, but a vast improvment on the convential sleeping bag! they are a little expensive at the moment and come in alot of colours, there are also 3 versions of increasing price which offer lower temperature range, lighter weight, better fabrics and additional colours.