Tribunas. Los artículos estrictamente de opinión (tribunas) responden al estilo propio del autor y no serán retocados, salvo por razones de ajuste o errores flagrantes (incluidos los ortográficos).
http://www.saltana.com.ar/biblib/files/pais.pdf

This edition takes a broader view of opinion writing than earlier editions did. Traditional newspaper and broadcast editorials are thoroughly studied, as in the past, but I have also given more attention to the growing opportunities to write cultural reviews, ***signed opinion pieces***, columns, interpretive articles, “public journalism” articles and Internet contributions.
http://www.stratapub.com/Rystrom 4/preface.htm

Other refs:
http://www.google.es/search?num=100&hl=en&q="signed opinion ...

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Note added at 36 mins (2005-05-24 17:12:58 GMT)
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Probably better known as op-ed pieces, on second thought--over 90,000 refs.:

While the Polly Klaas Foundation may be new to the experience of writing letters and filing ***op-ed pieces, as signed opinion articles are called***, other charities have for many years used such writings to reach potential supporters.
http://philanthropy.com/jobs/2003/05/29/20030612-885735.htm

Tons of refs. here:
http://www.google.es/search?num=100&hl=en&q="op-ed pieces"