Jul 14th 2009 3:34PM Biff~
Muti Rogues typically have Instant on their main hand and Deadly on their off-hand, both for poison specialization and their interaction with Envenom finishers. Instant poison doesn't really become viable until the poison buffing talents are taken in the Assassination tree. Until then, it doesn't proc often enough to do more damage than wound but once it does, it slides past it nicely.
TLDR: Deadly Poison provides "Poisoned target" for you.