This information is dragged from a couple of threads; there's a lot of discussion on haste atm, so I thought I'd stick it in it's own. A little presumptuous perhaps, but if it's of no use I'll just delete it and ban myself for spam...
If there's more information that should be in here, or if anything needs correcting, say so.
With 4.01 haste buffs have changed. Raid buffed you used to be able to expect a 3% buff from Improved Moonkin Form and 5% from Wrath of Air Totem, requiring you to have 1269 haste to reach 50% and cap your gcds. Now Balance Druids, Shaman not using Windfury and (I believe) Shadow Priests can all provide 5% haste, but they don't stack.
This means that caps for GCD and backdrafted Incinerates are now as follows:
Note that ISF and Heroism do not stack.
Normal With Improved Soul Fire With Hero/BL/Time Warp
GCD Cap (50% haste): 1406 795 325
Backdraft Incinerate: 1640 997 505
The new DoT mechanics allow your dots to benefit from haste, reducing the time between ticks and adding additional ticks if they fit into the spell's duration. That duration can fluctuate in between d - p/(2(1 + h)) and d + p/(2(1 + h)), where d is the base duration and p is the base interval between ticks. This means that:
Assuming 5% haste buff these are required haste ratings to gain ticks for various dots. At each point below the spell will be at it's maximum duration, requiring fewer gcds to be spent on refreshes, and drastically increasing the value of haste for that spell. In the case of Demo, this also means you have the most chance of successfully managing to sustain Immolate with HoG. In the case of Destro, extra ticks mean more Immolate damage and hence bigger Conflagrates.
n = (d + p/(2(1 + h)))(1 + h)/p = d/p(1 + h) + 1/2
h = (n - 1/2)p/d - 1
edit - corrected
d = duration
p = interval
h = haste (as fraction or decimal)
n = number of ticks (only works if n > than base number of ticks)
Bane of Doom is unaffected by haste.
Ticks GoBoA BoA Corr UA Immo
+1 0 0 105 157 157
+2 179 235 625 781 781
+3 402 495 1146 1406 1406
+4 625 755 1666 2030 2030
+5 848 1015 2186
+6 1071 1276
+7 1294 1536
+8 1517 1796
+9 1740 2056