You guys are confusing the Tube map (LU + DLR + North London Line)
with the Connections map, showing all passenger raill ines in London.
TfL has also released a 2010 Tube Map earlier this year, clearly
showing that the whole Overground would appear on the map in orange.
http://www.bbc.co.uk/london/pdf/tube_map2010.pdf
See also: http://en.wikipedia.org/wiki/London_Overground