http://en.wikipedia.org/wiki/Helicopter_prison_escapes
2.1 Oct, 1973 - Mountjoy Gaol, Ireland
2.2 Jan, 1983 - Melbourne, Australia
2.3 May, 1986 - La Santé Prison, France
2.4 Nov, 1986 - Federal Correctional Institution at Pleasanton, USA
2.5 Dec, 1987 - Gartree Prison, Leicestershire, UK
2.6 Aug, 1989 - Colorado prison, USA
2.7 1990 - Federal Holding Facility, Miami, FL, USA
2.8 1991 - Las Cucharas prison, Puerto Rico
2.9 1992 - Prison near Lyon, France
2.10 Sept, 1997 - De Geerhorst jail, Holland
2.11 March, 1999 - Metropolitan Remand and Reception Centre,
2.12 2000 - Prison near Lyon, France
2.13 2001 - Luynnes prison in southern France
2.14 Mar, 2001 - Draugignan prison, France
2.15 May, 2001 - Fresnes prison, south of Paris, France
2.16 Jan 2002 - Parada Neto Penitentiary, Brazil
2.17 Dec, 2002 - Las Cucharas prison, Puerto Rico
2.18 2003 - Luynnes prison in southern France
2.19 July, 2005 - central France
2.20 Dec, 2005 - Aiton Prison, France
2.21 June, 2006 - Korydallos Prison, Greece
2.22 April, 2007 - Liege, Belgium
2.23 July, 15 2007 - Grasse prison south-east France
2.24 Oct 28, 2007 - Ittre, Belgium
Whirlybird, I'd say they've thought of it before
Most of the prisons by the look of it focus primarily on prevention; anti helicopter netting or wire over places a heli could land, etc. I too thought of it as part of a book a while back but I believe it was already in one; something tells me a Frederick Forsyth novel maybe?