This poison is VERY devastating at high levels (70+) if your gear isnt the greatest, and gets to the point where you take 100+ damage every second if there are enough in a swarm.
It is of note that poison does not currently turn your health bar green in SMP. This will probably be addressed once the existing DOT system is expanded to include things like slow-effects and other debuffs.
Another thing to point out is that although it is most likely to be a bug, scorpions are able to kill the player on creative mode, so the player should be wary of scorpions around them. However, scorpions take quite a long time to kill the player on creative mode, so if you are in a dungeon for example, the best thing to do woud be to fly to the ceiling, use a bow and arrow to kill the scorpions and then destroy the monster spawners.