I'm adding information that I posted on EJ Affliction Warlock thread to help clarify this point:
for reference: http://elitistjerks.com/f80/t112939-affliction_cataclysm_dots_you/#post1834888
To clarify some terminology for non-locks:
ISF (improved soul fire) -15% haste
Dark Intent - 3% haste
UA - unstable affliction
If you have 580 haste on gear, your haste is:
ISF + Dark Intent + 5% buff + gear (580/128.05701) = 30.006% haste
If you have 581 haste, your haste is:
ISF + Dark Intent + 5% buff + gear (581/128.05701) = 30.015% haste
The interval of UA is the base, 3 seconds, divided by haste:
580 Haste Rating: 3/1.3006 = 2.30759 seconds
581 Haste Rating: 3/1.3015 = 2.30742 seconds
In a 300 second fight you will get:
580 Haste Rating: 300/2.30759 = 130.006 UA ticks, or, 130
581 Haste Rating: 300/2.30742 = 130.015 UA ticks, or, 130
You get an extra tick on UA at 581 haste rating, but the damage you gain from UA doesn't jump.
The only real "jump" is that the duration of UA at:
580 haste = 13.85 seconds = cast UA 22 times
581 haste = 16.15 seconds = cast UA 19 times
Cast time is 1.15 seconds, so, you gain a total of 3.45 seconds over the course of a 300 second fight. You can't even cast two shadow bolts in that time.
I think this is a very interesting mechanic, but, in the end, saying these "thresholds" are somehow important to our DPS as affliction warlocks or shadow priests is very misleading. We're already going to be tweaking our gear to have as much haste as possible. Would you really reforge away 50 crit or hit to squeeze in an extra shadow bolt? Would you change a 20int/20haste gem to 40 haste? Would that even be a DPS gain? The numbers we are talking about here fall within the margin of error in SimC, so it is hard to test.
Jmickey on elitist jerk's forum estimated about a 50 DPS "extra" gain for going from 580 to 581 haste. I would concur that this is approximately the value of the gain. Also keep in mind that this is the maximum value of the gain - each point of haste above 581 diminishes this "extra" DPS gain for hitting a threshold.