Rewriting the old post updated with new scalefactors, I'll use 2 different value models for hit. The first one being as I did in the last post (1 hit = 1 dps, because you'll overcap hit a lot by valueing it higher), the second value being the gem conversion rate (If you have an amount of hit on your trinket, you can remove hit gems and replace them with spellpower gems, therefor the value of hit on trinkets can be said to be roughly [hit/20*23*Spellpower scale factor] if you need hit rating.
Formule: Castfrequency / Proc chance = Time to proc.
25% proc chance with a 1.5s cast frequency means 6 seconds to proc after cooldown is up.
15% proc chance with a 1.5s cast frequency means 10 seconds to proc after cooldown is up.
10% proc chance with a 1.5s cast frequency means 15 seconds to proc after cooldown is up.
Internal CD = 45s.
Benefit from proc = Proc / (Internal CD + Time to proc) * Proc Duration
Average haste gain throughout the proc of Fetish of Volatile power shown
here, the ilvl 245 version would be /57*64.
Scale factors (as of 02/09/09):
Warlock_T8_00_13_58 Sta=0.00 Int=0.31 Spi=1.02 SP=1.59 Crit=0.82 Haste=1.10
SP/Haste/Hit/Crit = How much of that stat you gain averaged out over long fight.
Proc = Proc chance
DPS = Dps value of the trinket assuming 1 hit = 1 dps, explanation for this above.
DPSg = Dps value of the trinket according to the gem stat conversion value also explained above.