Update 29/11/09: So many people ask how to do this knot as it is tricky to learn at first - here's a link as well to a useful annimation which ties in very well with what I am doing in the video:
http://www.k2esu.org.uk/friendshipknot.gif
and a link to some instructions you can print off.
http://guidinguk.freeservers.com/necker.html
In the video I am using my Hermes Carnevale de Venise 90cm silk twill scarf.