Poisons CAN crit, this is correct. But as this only happens once in a lightyear and probably is only an oversight on Blizzards side, i think to calculate the crit rate of a Rogue poisons should not be included in the calculation. Aside from poisons, Blade Flurry can definatly not crit (or i have been extremly unlucky in the past 2.x years with this talent :) ).
I have been playing a Rogue since alpha and can't say I have ever seen poison crit. Of course, I don't really watch for it either so I guess it is possible.
You're just not watching for it. I only have about 20 days played on my rogue and have seen plenty of poison crits.