Cho'gall is an interesting fight for us.
To answer your first question, yes, use Mind Sear on the festers. It used to be that we were completely useless on that phase, and would just stay on the boss while everyone else AoE's it down, but Mind Sear is (*gasp*) actually useful now! In fact, an undocumented change (or possibly a bug) now causes it to damage the target as well, if its hostile.
For breaking fear, I usually will Scream if the person is close to me, otherwise I'll let someone else get it. By my logic, in the time it takes me to run all the way over to the person and Scream, someone else is likely to have already broken it (assuming your raid knows what it's doing. For progression, you should always break someone out if you see nobody else is getting it. Less dps on the boss is always preferable to him getting a full worship stack). I am not, however, specced into Horror or Silence. So that might be different if it's just a quick cast. In that event, I would recommend going ahead and breaking it. If your healers are good, they should dispel the fear effect, and in fact might just be thinking they're dispelling Corruption: Accelerated
For the burn phase, multi-dot here. Abuse the hell out of that mechanic. Combine that with our execute, and you should begin to soar in the dps meters here. I seriously love this phase, from the epic intro speech to the giant numbers I can pump out there. I would keep DoTs up on everything, but use Blast and Flay (and probably DP) on the Creations. This means they get burned down one at a time, while still maintaining dps on the boss and other Creations around them.
It's a fun fight when everything goes right, but there's little room for error. Best of luck! Let us know how it goes. Edited, May 26th 2011 1:34am by IDrownFish