I'm not really sure.
I started sharing private versions in april 2010 (would have been TheRob) and I recall that i started forking some desktop shell called something like "razer" by Ian Reinhard Geiser (which itself contained many elements of KDesktop 3.x) in "september" - non of that code is meanwhile left, though. I rewrote most of it within the first 3 weeks. Later (when having shared it with too many ppl. in private) took a closer look at the taskbar ... and systray code
So I assume it was September 2009. In particular the day where all systray icons vanished in plasma
PS: if you wonder - I assume razor-qt was forked from Ians code as well.