I have heard the term used for about 20 years in various contexts, but all of the conclaves I've attended have been flyfishing gatherings. It is a common term coined in fun, kind of like saying a highly regarded destination is "mecca".
Here's a great place to learn the inside scoop about words...
http://rhyme.lycos.com
The definition is "a confidential or secret meeting". Many of our gatherings are publically arranged online but the exact whereabouts, conditions, and techniques are eyes only - in other words if you join us in person you get the inside story.
Clubs of every interest group have used the term to describe their events, for instance if you search on the term you'll see that it has been used by every type of club from computer gamers to golfers to pipe smokers, etc. It's really a widely used universal term to say "let's gather to celebrate or interact directly within shared interests".
I think Dana hit the nail right on the head by calling his discussion area the Spey 'clave'.