In the US this is called 'chain broadcast'. The following extract is from http://caselaw.lp.findlaw.com/cgi-bin/getcase.pl?court=us&vo...

[ Footnote 1 ] Chain broadcasting is defined in 3(p) of the Communications Act of 1934, 47 U.S.C.A. 153(p), as the 'simultaneous broadcasting of an identical program by two or more connected stations'. In actual practice, programs are transmitted by wire, usually leased telephone lines, from their point of origination to each station in the network for simultaneous broadcast over the air.

More generally, the term '(nationwide) simultaneous broadcast' is used, as here:

DefenseLink News Article: Sailor's Mom, Friends of Military ...That was the day hundreds of radio stations nationwide and sailors and Marines ... "We'd like to do another simultaneous broadcast of the National Anthem on ...
www.defenselink.mil/news/newsarticle.aspx?id=29294 - 49k - Cached - Similar pages

Attacks on the Press 2003: AmericasChávez continued to use cadenas—nationwide simultaneous radio and television ... to determine whether they had violated media broadcast regulations. ...
www.cpj.org/attacks03/americas03/ven.html - 15k - Cached - Similar pages

CPJ News Alert 2007This would compel RCTV to broadcast a certain amount of nationally produced ... cadenas—his nationwide simultaneous radio and television broadcasts—live. ...
www.cpj.org/news/2007/americas/ven31july07na.html - 9k - Cached - Similar pages


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Note added at 20 hrs (2007-11-14 13:24:13 GMT)
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In response to the various doubts about this term, I would add the following:
I've known the expression 'cadena televisa' in Latin America since the mid 1970s when I worked at local radio stations in Honduras; there was (perhaps still is) a law that required all stations to carry certain programmes originated by the government. I was personally involved in organizing a 'cadena radiodifusora' in Guatemala following the February 1976 earthquake. In those days (and perhaps even now in some countries...) the stations set up these 'cadenas/chains' simply by picking up and re-broadcasting the signal from their nearest neighbour - in much the same way as kids join hands to form as human chain.

The description given by antsoa corresponds exactly to this concept: the interlinking of all the country's stations to carry a common programme - most commonly a political address or some kind of propaganda (including 'Party Political Broadcasts' during election time in the UK, for example, so it's not restricted to Latin America).

Without getting into politics here, we may note that one frequent user of this medium is the President of Venezuela, as mentioned in the links above.