Spanish term (edited): del dicho al hecho, hay mucho trecho

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Note added at 5 mins (2009-06-20 01:47:11 GMT)
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depending on the context.

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Note added at 6 mins (2009-06-20 01:47:55 GMT)
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put your money where your mouth is

I think that's one popular equivalent.

No reason to complete the expression, as it is so widely-used the three dots can be used, and to particularly good effect in a headline.

("...of mice and men / gang oft agley" as Burns said - "go oft awry")

I believe the SP expression is emphasizing the possibility of unforeseen complications, rather than disingenuous intent, as a couple of the above suggestions seem to take it.)

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Note added at 3 hrs (2009-06-20 05:34:40 GMT)
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should be "widely used"

Saying is one thing; doing is another - Idiom Definition ... - [ Traducir esta página ] Saying is one thing; doing is another - English Idiom Definition.
www.usingenglish.com/.../saying is one thing; doing is anot... - En caché - Similares


Phrases and Expressions in Spanish | Planeta Del dicho al hecho hay mucho trecho. From the word to the deed, there is a great distance. 'Actions speak louder than words' ...
www.planeta.com/ecotravel/.../dichos2.html - En caché - Similares

Quizás te sirva.

Economist's View: "There's Many a Slip Twixt the Cup and the Lip" - [ Traducir esta página ]Larry Summers says fiscal policy needs to be part of the policy package that is implemented to try to avoid a recession. As I've noted several times ...
economistsview.typepad.com/.../theres-many-a-s.html - En caché - Similares -

there's many a slip 'twixt the cup and the lip definition ... - [ Traducir esta página ]Definition of there's many a slip 'twixt the cup and the lip at Dictionary.com with free online dictionary, pronunciation, synonyms, and translation.
dictionary.reference.com/.../there's+many+a+slip+'twixt+the+cup+and+the+lip -

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Note added at 10 mins (2009-06-20 01:52:34 GMT)
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http://economistsview.typepad.com/economistsview/2008/01/the...

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Note added at 12 mins (2009-06-20 01:54:01 GMT)
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http://en.wikipedia.org/wiki/There's_many_a_slip_twixt_...

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Note added at 21 mins (2009-06-20 02:03:42 GMT)
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Second link seems to be working badly. Sorry

I stand corrected: 'twixt... Thanks, Mediamatrix!

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Note added at 11 hrs (2009-06-20 12:51:09 GMT)
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Wikipedia:

There's many a slip 'twixt the cup and the lip is an old English proverb. It implies that between the time we decide to do something and the time we do it, things often go wrong.

A Latin form is found in Erasmus's "Adagia," I.iv.1 ("Multa cadunt inter calicem supremaque labra") which appears to derive from an epigram by Palladas in "The Greek Anthology" (X, 32).

The proverb supposedly comes from a Greek legend in which one of the Argonauts returns home to his winery. A local soothsayer had previously predicted the Argonaut would die before he tasted another drop of his wine, thus the Argonaut calls the soothsayer and toasts him for the Argonaut had survived his journey. The soothsayer replies to the toast with a phrase corresponding to the English proverb. As he finishes his toast, the Argonaut raises a cup filled with wine to his lips but is called away to hunt a wild boar before he could take sip. The Argonaut is killed hunting to boar.[1]

The first occurrence of the proverb in English, according to the Oxford English Dictionary, is in Thackeray's Pendennis, 1850.[2]

From Murphy's laws for managers.
The correct translation from the original would probably be to "talk the talk, but not walk the walk". However this may be hard for you to fit in the context.