Carrel.ORG – GPGMail and Mac OS X 10.4.1: http://www.carrel.org/2005/05/16/gpgmail-and-mac-os-x-1041/
Excellent: works a charm !
UPDATE: Well, maybe not. Just had Mail die during a composition when trying to change the quote level, and upon restart it told me I no longer have a GPG key, and my draft has vanished entirely… Not ideal behaviour, really: maybe it was disabled by Apple for a reason ?