If you count everything as PDA's (Treo's and LifeDrive's) then the Canalys numbers are surely what you are after. If it can run apps and fit in your hand they count it.
Looking at those numbers, despite the Treo, Palm OS is still down year on year, and is only 37% of the WM VS POS market, with WM the other 62% of course.
Still, more than 3 million units are being sold by traditional PDA makers (POS and WM), coming to >12 million per year, which matches or tops 2001 certainly.