I know Watha and I have been wondering this, and didn't know if the rest of you were.  According to the Warlock's Den, your trinkets affect your spells when they are cast, not DOT's that are already up, so the best time to hit them is right before you start casting so that each spell you cast will benefit from the trinket.  Kind of makes CoD interesting too doesn't it?

for those of us who remember to use CoD *coff*

Or aren't the CoE bi***