Actually, the quote is correct as is, although the version you mention is all over the Internet as well.
Algernon says the line in response to Jack in Act 1 of The Importance of Being Earnest...
"The truth is rarely pure and never simple"
(Importance of Being Earnest, Act I).
http://www.nybooks.com/articles/13019
The exchange goes like this:
JACK: My dear Algy, I don't know whether you will be able to understand my real motives. You are hardly serious enough. When one is placed in the position of guardian, one has to adopt a very high moral tone on all subjects. It's one's duty to do so. And as a high moral tone can hardly be said to conduce very much to either one's health or one's happiness, in order to get up to town I have always pretended to have a younger brother of the name of Ernest, who lives in the Albany, and gets into the most dreadful scrapes.
That, my dear Algy,
is the whole truth pure and simple.
ALGERNON: The truth is rarely pure and never simple. Modern life would be very tedious if it were either, and modern literature a complete impossibility!
~from The Importance of Being Earnest, Act 1, by Oscar Wilde
Check out this link to a description of a MUST SEE movie about Wilde called "Wilde"
http://www.oscarwilde.com/wilde.html