Thanks all, decided to google/wiki the phrase and look what I turned up...
http://en.wikipedia.org/wiki/Illegitimi_non_carborundumIllegitimi non carborundum is a mock-Latin aphorism jokingly taken to mean "don't let the bastards grind you down". There are many variants of the phrase, such as
Nil illegitimi carborundum.
Non illegitimis carborundum.
Illegitimi nil carborundum.
Non illegitimi carborundum.
Nil bastardo carborundum.
Nolite te bastardes carborundorum.
Illegitimis non carborundum.
Illegitimus non carborundum est.
Nil illegitimo in desperandum carborundum
None of the above is correct Latin. Carborundum is not a Latin word but the name of a mineral which is extremely hard and used for grinding. (see silicon carbide article). The ending -undum suggests either a Latin gerund or gerundive form--and the idea of obligation ("Don't let ...") is more suggestive of the gerundive--but the word is actually a portmanteau of "carbon" (from Latin), and "corundum" (from Tamil kurundam).
Illegitimi suggests illegitimate to the English speaker, or bastardo likewise, but the Latin for bastard is actually nothus (from the Greek word notho (νόθο) meaning not-pure, and used when referring to a bastard whose father is known) or spurius (for a bastard whose father is unknown). The forms with nil may be formed partly on the pattern of the genuine Latin phrase Nil desperandum.
The phrase originated during World War II. Lexicographer Eric Partridge attributes it to British army intelligence very early in the war (in the plural illegitimis). The phrase was adopted by US Army general "Vinegar" Joe Stillwell as his motto during the war.[1] It was later further popularized in the US by 1964 presidential candidate Barry Goldwater.[2] The United States submarine USS Tunny (SSN-682) uses it as the ship motto. The weekly Alaskan newspaper 'The Nome Nugget' uses it as a motto, as does the Whitehorse Daily Star, in the capital of the Yukon Territory.
Henry Beard in his 1991 book Latin for Even More Occasions (chapter I) offered some correct Latin for the sentiment, but did so in a section "Dopey Exhortations Are More Forceful in Latin", which might be his comment on the merit of the expression.
Don't let the bastards wear you down.
Noli nothis permittere te terere.