Happened to me too. I had crazy issues with my 1520. Actually sent mine in and the repair center sent it back to me with no repairs completed because I was running the dev preview. I simply flashed WP8 back onto it and sent it back in. All was good after that.
I spoke with a rep there who told me that in the agreement for the dev preview, it states that the warranty is indeed voided while running WP8.1 preview. She also said that the warranty is reactivated once the official release is updated to your phone running the preview. I never actually verified to see if the agreement says anything about the warranty, but it's probably really in there somewhere.