Here's a fairly detailed report with several screen shots.
I play Horde on the Sentinel server, I run a custom built computer with an AMD processor, and an Nvidia GeForce 9800 GT video card. There are only two areas where this happens to me that I have noticed. The first is Uldum, where you first port in when taking the port from Orgrimmar. I have attached pictures on my DK as well as my shadow priest. the results of the glitch are different for the toons. On my DK it is only the weapon runes that are being displaced. this is not the only place in the zone this is happening, but to list them all would take too long and you said you didn't want all that. One thing I find confusing about the displacement issue with the viewport is if I simply change my view or rotate my character it looks normal as if nothing is broken. I have also attached pictures of this.
the second notable area the viewport is glitching is in the throne of the tides instance, the five player dungeon. this time it is the whole instance that this problem exists and also the same thing where if I simply change my view or rotate my character in certain places, like the entrance where you initially port into, I see everything "normally". I have attached several pictures to this effect.
I have not done any raids yet as I had taken a break from wow and am only now playing again, so I don't know if this happens in raid zones.
one other thing I'd like to say here is that when I type the /nui viewport command, it always displays the message that viewport has been disabled, even when I know it is Enabling it.
the consistent aspect is that when I disable the viewport these problems do not happen. I have taken a couple pictures where I have my UI hidden so you can see I have disabled the viewport and enabled it. hope this helps.
Just want to say also, love the UI and hope the work with the viewport gets resolved quickly so you can stop stressing over it. I appreciate the hard work you put into this and thank you for creating this wonderful addon.
What people don't get is that I am, ultimately, an artist at heart. My brush has two colors, 1 and 0, and my canvas is made of silicon.