To clarify:
The concept of Proc Per Minute means that the chance to proc on a successful melee attack is a function of the weapon's speed. If say Berserking has a proc rate of 1.2PPM, then your chance to proc (unhasted) on any given landing attack is 1.2/(60/WeaponSpeed), or in words, the PPM divided by the number of swings you make in a minute. I believe that the chance to proc Berserking is effected (affected?) by haste effects, so your chance to proc goes down as you gain more haste but the average number of procs you get stays the same.
My data is showing that poison procs are normalized around the idea that they should have the 3.0.8 proc rate if you are using 1.4 speed weapons. This means that if you use Wounding Poison with a
[Webbed Death] you will proc it on 50% of all landing attacks, and likewise will proc Instant Poison 20% of the time. When you use a weapon that is slower than 1.4, your proc chance goes up -
[Sinister Revenge] will proc WP at 1.8/1.4*.5=64.29% and IP at 1.8/1.4*.2=25.7% chance. This goes up to using a 2.8 speed weapon -
[Warglaive of Azzinoth] will proc WP at 2.8/1.4*.5=100% and IP at 2.8/1.4*.2=40% chance. This particular proc chance seems unaffected by haste, so the proc chance will not go down as you gain more haste, and the average number of procs you get increases.
I'm not sure about Mongoose or Berserking, but special attacks like Sinister Strike are procing poisons at a chance equal to white hit proc chance. A 2.8 speed mainhand will proc Wounding Poison at a 100% rate (minus some small percentage, my small test run of 500 hits showed 2 non-procs). Going dual IP with SR/WD for mutilate means that the mainhand will proc IP 25.7% of the time and the offhand will proc IP 20% of the time (not counting talents, multiply the chances by 1.5 to account for Improved Poisons, which is apparently currently broken).
The bottom line for the Combat spec is that you gain damage, period. If poison damage or the scaling coefficients haven't changed, then Wound Poison will still be the mainhand untalented poison of choice. This model also vastly prefers slower weapons in the main hand for several reasons: the number of Sinister Strikes you perform is independent of mainhand weapon choice, the damage of your Sinister Strike is higher with a slower weapon, and poisons will proc more often on Sinister Strike with a slower weapon. There is no upside to using a faster weapon in the mainhand that I can see. In the offhand, however, faster weapons will still be preferred. Other than Killing Spree you don't do any instant attacks with the offhand, so if you use WP or IP in the offhand the number of poison procs you have in a minute is independant of weapon speed. However, Combat Potency accounts for a large amount of your energy generation, so you will still want a very fast weapon in the offhand to proc it more often. Since you're using a fast weapon in the offhand, you will use Deadly Poison (a non-PPM effect that is already better than WP at its 3.0.8 proc rate) to complement it.