8 pages, 8 figures, uses edp-jp4.cls. Invited talk at the Workshop on
  "\emph{Complex Systems: New Trends and Expectations}'', Santander (Spain) 5-9
  June 2006. To appear in The European Physical Journal Special Topics, Vol.
  143 "Trends and Expectations in Complex Systems Science'', Eds. H. S. Wio, L.
  Pesquera and M. A. Rodr\'{\i}guez