Same. My depression was in remission for a good while, and then a few months after my DP set in, so did the depression. It all came back, and very strongly. To answer your question 2deepathinker, yes, I believe that depression can cause/lead to derealization. I've read that in a literature somewhere. It can also occur comorbid alongside DP/DR.
Edit: Also I find that my DP and depression follow a vicious cycle. The more depressed I am, the stronger the DP, and the stronger the DP, the more intense depression. Messed up...im telling you.