Who said, "A man is half smart when he admits he knows nothing"?

This sounds like a loose paraphrase of words attributed to Socrates.
You might find some interesting reading in my rejected answer to this
question:

http://www.answers.google.com/answers/threadview?id=363671

Pinkfreud, I was taking a look at the old question you linked to. 
When you were researching it, did you ever come across the following
line from Plato's Charmides?

"Then wisdom or being wise appears to be not the knowledge of the things
which we do or do not know, but only the knowledge that we know or do not
know?"

I would imagine that a more modern translation would yield the
wide-spread quote.  (Which I think was going around before Bill &
Ted's.)

You can find the full text here:
http://gutenberg.net/dirs/etext98/chmds10.txt

There are many similar things in Plato, but the customer who posted
that question wanted a certain brief, snappy wording which I have
found nowhere except in the utterances of Bill & Ted. ;-)