I'll explain later. gtg :-)

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Note added at 2 hrs (2009-07-27 22:50:55 GMT)
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I forgot the "trabajadores" part, so I correct it to:

"Single Income Tax"

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Note added at 3 hrs (2009-07-27 23:51:16 GMT)
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The description says "... grava las rentas percibidas por los trabajadores dependientes del país aplicando a la remuneración una tasa progresiva", so, this tax will use a progressive rate, so it is not correct to use "flat rate" because flat rate is the same as fixed rate. It is not a fixed rate tax, it is a progressive tax.

http://en.wikipedia.org/wiki/Flat_tax
http://en.wikipedia.org/wiki/Progressive_tax

Since it is a tax that will be applied to the work of an employee, it is an income tax.

http://en.wikipedia.org/wiki/Income_tax

Uno impuesto único is a single tax, the only tax applied to a certain situation.

Examples:
http://www.goarticles.com/cgi-bin/showa.cgi?C=1282655

In Brazil, the concepts are the same:
http://www.brazilexporters.com/dicti.html

"...
Imposto diferido - deferred tax
Imposto fixo - flat tax
Imposto progressivo - progressive tax
Imposto regressivo - regressive tax
Imposto sobre a renda - income tax
Imposto sobre valor adicionado - value added tax
Imposto único - single tax
..."

Google:
http://www.google.com/search?q="single income tax"&hl=en&cli...

HIH

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Note added at 8 hrs (2009-07-28 05:19:17 GMT)
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I need to make a correction, actually a BIG correction. There is a piece of information in your description that is essential, the word "dependiente". If we talk about "income tax", it is the tax paid by workers in a general sense, but "trabajo dependiente" is used for employees that have a link with the employer, or "trabajo asalariado":
http://es.wikipedia.org/wiki/Trabajo_(Derecho)

In English, "trabajador asalariado" is "wage earner".

So, "impuesto único al trabajo dependiente" is actually "income tax on wage earners". About the "único", my studies showed me that, in Chile, the "único" meant that this income tax was used by wage-earners that originally had only ONE source of wages, i.e., one only employer. It was supposed to be a simple tax with simple forms. Although today it is the tax applied to wage-earners that have more than one source of wages, it is still called "único". Translating this "único" using "single" would actually lead to misunderstandings since "single tax" has a different concept.

The previous answer, "income tax paid by workers" is not a bad answer but since "workers" can imply not only wage-earners but also a self-employed professional, I would strongly advise you to use "wage-earners" that is surely the translation for "trabajador dependiente", for this specific context.

Please, see:
http://es.wikipedia.org/wiki/Trabajo_(Derecho)
"Actualmente la forma dominante es el trabajo asalariado, o trabajo en relación de dependencia, o trabajo por cuenta ajena. El trabajador o "empleado", es reconocido en su condición de persona al igual que aquel que va a utiliza su trabajo ("empleador")."

Some google links for income tax on wage-earners:
http://www.google.com/search?hl=en&client=firefox-a&rls=org....

http://www.google.com/search?hl=en&client=firefox-a&rls=org....

Some literature about Chilean tax system:
http://www.sii.cl/aprenda_sobre_impuestos/impuestos/imp_dire...
Read about "Impuesto Unico de Segunda Categoría (Artículo 42 N°1 y Artículo 43 N°1 Ley de la Renta)"

Summing it all up, please, forget my previous answers and consider this one:

Income Tax on Wage-Earners.

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Note added at 8 hrs (2009-07-28 05:25:24 GMT)
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I forgot this link: http://www.babylon.com/definition/asalariado/

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Note added at 2 days7 hrs (2009-07-30 04:17:13 GMT) Post-grading
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You're welcome.
Thank you too.
AV