They have afterburners? Might have just found my 5th pet then
Other poison DoT's do stack (in fact, look at the massive nerf they had to hit scorpids with a few patches ago) but I guess that there will be the usual limit of 5, it won't scale particularly well, and it might well eat focus. I was just thinking that the armour reduction was most useful given level 80 bosses will have re-adjusted armour values and a lot of Wrath of the Lich King gear has ArP on it.
Remains to be seen, but I kinda like the idea of afterburners and the annoying sound they make anyway.