What is known about Darth Vader's chest plate is that the inscriptions are written in Hebrew.  In the past, fans only had theories and guesses as to what this inscription said.  But now, thanks to TheForce.Net and their Rebel Rouser, they've brought the fans an answer.  According to them, "out of the West, rides a lone Newbie," Anat, someone who speaks Hebrew , who gives her best shot at a translation.  The translation that appears here has been edited by me, for sake of organization and order, but the original can still be viewed here, in its entirety 

     These lines, which together form one sentence, are written in an ancient / Biblical style and syntax, as opposed to Modern Hebrew style and syntax.  In Hebrew, like many other ancient and modern languages, words often have several meanings, regardless of the style, grammar, or syntax in which they are used, and these words are then defined according to the context of the sentence.  Therefore, translations can be quite tricky...  keeping this in mind, here is Anat's suggested translation:

The First Line:  Phonetically pronounced Ein MaaSaV - suggested translation - "His deeds will not."

The Second Line:  Phonetically pronounced NimChaLim - suggested translation - "Be forgiven."  Note:  the inscription of this line is placed upside down, with the letters being partially covered. 

The Third Line:  Phonetically pronounced Aad ShezaCha - suggested translation - "Until he merits."

"Ein MaaSaV NimChaLim Aad ShezaCha"

"His deeds will not be forgiven until he merits"

     Combining these lines, you get the complete statement of "His deeds will not be forgiven until he merits," which fits the persona of Anakin Skywalker / Darth Vader.  However, what makes this matter more interesting is, was this meant to be part of the story, or just a mere anecdote? 

     The last word of the third line, "ShezaCha," can also be read as "Shezooka."  Hebrew is read from right lo left, and the word is composed of four letters:  Sheen, Za'yin, Chaf, and Hay.  "She," at the beginning of a word often means "that" - the word "that" is simply glued to the word, and is not a separate word as it is in English.  Therefore, since "Zacha" means "won" or "has won", "ShezaCha" means "that has won."  Sheen makes the "Sh" sound and Za'yin makes a "Z" sound.  However, Hebrew has a system of dots and lines around letters that work similar to vowels.  They tell the reader if a Sheen should be She, Sha, Shee, Shoo, Sho, etc., and it also informs them of the actual sound the letter makes.  For example, Sheen has a variation called Seen - if the dot appears on the upper right, it will be a Sheen, and if it appears on the upper left, it will be a Seen.  In Biblical Hebrew, this vowel-system is very important; the inscriptions on Darth Vader's chest plate are clearly written in Biblical Hebrew, however, no punctuation exists.  The word "Zooka" is passive, and can be translated to "has been proved innocent" - the similarity to "Zacha" is evident.  Normally, "Zooka" would be written with the following letters: Za'yin, Vav, Kaf, and Hay, with Hay being a vowel letter - this would tell the reader that the word is not "Zacha," which is written using the letters Za'yin, Chaf, and Hay.  However, it often occurs that quoted Biblical text is stripped of punctuation, but not added vowel letters, therefore often becoming unintelligible (and hence the two translations provided).  So, the word "Zooka" can be written without a Vav, using the proper punctuation.   If the third line is read "SheZooka," the translation would be "His deeds will not be forgiven until he is proven innocent," which is a reversal of one of the Mitzvot, which says "One shall be regarded innocent until he is proven guilty," which also adds to the inscription's Biblical air.