and yeah, diminishing returns exists, here's some info:

<from 2.3 patch notes under rogue header>
Blind is now a physical (no longer a poison) attack. Reagent requirement removed. Now shares the same diminish category as Cyclone and is now diminished in PvE as well as PvP.

<wowwiki page on diminishing returns, great info>
http://www.wowwiki.com/Diminishing_returns

<Addon that will track when cyclone can be used to full effect again>
Chronometer
Nature Enemy Castbar


Basically what this means however is that your first cyclone will last 6 seconds, 2nd 3 seconds, and finally 1.5 seconds. but that's only if you don't stack some other class spell that shares the diminishing return timer of cyclone such as rogue: Blind. Regardless of which druid does the actual casting.