Jay Wilson recently responded to a Twitter follower in regards to a question about how poison damage is going to work in Diablo 3. In his response(s) he indicated that poison damage is not going to apply a damage over time effect, by default. Instead, poison damage will apply immediate damage – just as with holy, fire, etc… However, he also stated that there will be abilities that will apply a DoT, including skills that will refresh the damage effect over a period of time.
So, it sounds as if poison damage will have a bit more burst in Diablo 3, rather than the “apply it, then run around and wait for things to die” effect that it had in Diablo 2. This will likely make poison damage modifiers just as popular as the other nature and magic damage effects.