Yes, today - July 23rd, 2008 - is the tenth anniversary of the release of Linux-Mandrake 5.1, the very first public release of what is now Mandriva Linux. It was versioned 5.1 as it was a fork of Red Hat 5.1 (it was more or less RH 5.1 with KDE 1.0 added).
Here is the original announcement in English, and here is the original thread, in French, from the fr.comp.os.linux usenet group. History will record that the very first comment on Linux-Mandrake was someone complaining that something wasn't packaged (xemacs) - so some thing never change...:) Funnily enough, I've been doing most of the maintenance on the xemacs package lately...
I did look for a usenet thread in English, but couldn't find one. Looks like the French one is all there was.
(We celebrated the anniversary of Mandriva itself earlier in the year - in May - but today is the birthday of Mandriva Linux, the distribution).