A lot of people don't make the distinction between DR and DP - indeed, usually the medical profession just lump it all under Depersonalisation. For me though, DR and DP were, although obviously related, entirely different experiences. I've always had a theory (in the loosest sense of the word) that DP is at the bottom of the anxiety ladder, and to get there you have to go through something like the following:
1.) Trigger/Cause - > 2.) Anxiety - > 3.) Panic -> 4.) DR -> 5.) DP
In my case, I went all the way to step 4 and got stuck there, thank god. Only on a few terrible occassions did I slip down to step five, and the dreaded DP. Some people don't even get that far, but spend their lives with, say, GAD, and flit between steps 2 and 3. I dunno though, it's probably too simplistic, but it works for me.