I wish to thank Gleb Arutyunov, Niklas Beisert, Sergey Frolov, Matthias Staudacher, Arkady Tseytlin
and Marija Zamaklar for helpful discussions and important comments on the manuscript. This review grew
out of a lecture delivered at the Post-Strings 2004 Meeting at Durham. I thank the organizers of the
meeting for hospitality and an inspiring workshop.