Dwiz - Super question. If go to Kilee's SP Guide here:viewtopic.php?f=19&t=340
Hit Ctrl+F and search the page for "3202" you will get the official answer. If you haven't already I would suggest reading the whole thing, its a fantastic piece of work.
Regarding some of the responses to Dwiz's post...
Haste does not increase damage done, it increases the frequency of damage done. This is a standard across all classes/specs in WoW. Haste works exactly the same for DP as it does for all other dots. Dot ticks in WoW are 3 sec, as you gain haste the tick time for your dot will decrease. For example at 36.93% haste, your tick time will be 2.19 sec. Dot duration's though vary quite a bit from spell to spell. Devouring Plague has a 24 sec duration (which in WoW speak means 24.999 sec). As you gain haste that total duration will decrease until it reaches a limit. At this point the dot is reset to its original duration and you get an extra tick of damage inside the total duration.
I did a quick test to show you what I mean, 85 Goblin in Shadow Form. This shows how haste affects Devouring Plague.
The first entry is naked, no gear:
H= 0000 = 09.23% = 09 Ticks = 2.75s Tick = 24.71s Dur
H= 0284 = 11.65% = 09 Ticks = 2.69s Tick = 24.18s Dur
H= 0558 = 13.99% = 09 Ticks = 2.63s Tick = 23.69s Dur
H= 0852 = 16.50% = 09 Ticks = 2.58s Tick = 23.18s Dur
H= 1028 = 18.00% = 09 Ticks = 2.54s Tick = 22.88s Dur
= 20.27% = 10 Ticks
= 2.49s Tick = 24.94s
H= 1522 = 22.21% = 10 Ticks = 2.46s Tick = 24.55s Dur
H= 1841 = 24.94% = 10 Ticks = 2.40s Tick = 24.01s Dur
H= 2137 = 27.46% = 10 Ticks = 2.35s Tick = 23.54s Dur
H= 2299 = 28.84% = 10 Ticks = 2.33s Tick = 23.28s Dur
H= 2447 = 30.10% = 10 Ticks = 2.31s Tick = 23.06s Dur
H= 2558 = 31.05% = 10 Ticks = 2.29s Tick = 22.89s Dur
= 32.30% = 11 Ticks
= 2.27s Tick = 24.95s
H= 3162 = 36.20% = 11 Ticks = 2.20s Tick = 24.23s Dur
H= 3247 = 36.93% = 11 Ticks = 2.19s Tick = 24.10s Dur
As you can see tick duration gets quicker at a steady rate as haste increases, and the dot duration resets every time an extra tick is added. It appears I get an extra DP tick at approx. 20% haste and again at approx. 32% haste. I didn't fool around with it enough to try to get a precise number.
This however does bring in to question the 1601 and 3202 numbers. As you can see I am definitely getting my extra ticks before those numbers are reached. I will assume for the time being that I am looking at something wrong until I can take the time to verify it more thoroughly.