While goofing off in a 0-2 shutout in WSG, I climbed on top of one of the center field stumps and began healing. I see a DK render in, run to me and try to DG me off the stump. I wasn't stunned, or incapacitated, so there was nothing I had done to make me immune...but I WAS. During the same match a Druid unstealthed behind me and opened up. One of my teammates tried to DG the Druid off the stump, and under the same circumstances (not stunned, etc.) the DG was Immune on its target. Does this have other possibilities with similar terrains in World PvP and BG's? Probably. Do I feel like /dancing on logs in AV to test it? Nah.
The red line w/ the arrow is the path that is used to get on top of the stump. If you look at my combat log, you can see he tried to DG me to no avail, so eventually he made his way up to the top and began hitting me like a kitten.