Hi Wally,
The Oxford English Dictionary lists this phrase under one sense of "all" 
and defines it as "Everything short of. Hence (adverbially) Almost, very 
nearly, well nigh." One of the OED's examples: "These were all but 
unknown to Greeks and Romans" . The earliest quotation given for "all 
but" is from 1598.
from:
http://www.phrases.org.uk/bulletin_board/9/messages/271.html

best wishes,
Ron

On 10/8/2011 2:41 PM, Day, Wally wrote:
> In regards to my previous post:
>
> I read the original comment "cancer was all but unknown" to mean "there was no cancer" just as Geoffrey did. That is the meaning that I have garnered from that particular phrase over the years. It is an incorrect statement, and that is why I agreed with G.P.


-- 
PK