Let me start this thread by stating that I've owned several LCD TVs recently (samsung, sony, a few others) and now am enjoying my 42" Panasonic Plasma (42PZ80U) as my primary gaming screen (mostly XBOX 360, some PC gaming).
What I have noticed with the Plasma (something I didn't initially even see with LCD - more on that in a minute) is that while running a game that operates at 30fps introduces "image/frame doubling." Bear with me as this is apparantly NOT the same as "ghosting" i.e. motion blur. When I first noticed this, shortly after firing up the Plasma and playing a game (GTA4 for 360 specifically), I thought it WAS motion blur/ghosting (I know this game incorporates motion blur - but this doubling effect is seen even when your character is standing still and you pan the camera in circles). But then I thought, wait a second, Plasma is inherently supposed to be much better than LCD when it comes to motion handling. As in no blur. What I was seeing did in fact look like blur. But upon closer inspection, it appeared that what I was seeing was not blur, but in fact image doubling. After some research (there was only a tiny bit of info about this phenomenon on the web, not much info to go off of), I decided to see if this was an effect of playing 30fps content on a 60hz refreshing screen. I did this by running World in Conflict for PC (this game allows numerous graphical tweaking, framerate capping, and a demo mode which reports your fps live and averages the framerate at the end of the demo). Now, my for my PC, I have a 24" widescreen LCD monitor with a 4ms response time. This is my baseline for comparison to the Plasma, as I don't have a CRT monitor/TV anymore. Results are as follows.
30fps on LCD @ 60hz: Moving images appear to double. Slight motion blur due to LCD response time.
60fps on LCD @ 60hz: Moving images are not doubled. Smooth movement. Slight motion blur due to LCD response time.
30fps on Plasma @ 60hz: Moving images appear to double. No motion blur detected.
60fps on Plasma @ 60hz: Moving images are not doubled. Smooth movement. No motion blur detected.
It appears that, using this game as a testing ground for different framerates, that any game (I assume other content as well) displaying at 30fps effectively has "doubles frames" for each screen refresh. This is done by the display device itself, and not the game/content. This is evident when fast moving images (full screen or otherwise)/objects are displayed on-screen at 30fps. I believe it is less noticeable on an LCD because of the inherent motion blur due to the nature of LCD technology. Since Plasma can refresh much faster than LCD, the frame doubling is more apparent, drawing each frame clearly hence showing the seperation of the image clearer than on an LCD. This is not ghosting because the "doubled image" is the same color as the original and is not a dark/fading silhouette of the image (clearly evident on Sony Bravia LCD TVs during fast-motion gaming, and some film/TV sources as well - this is due to the slower response time of these panels).
Also, I know what phosphor lag (green ghosting) is, and this is not the same thing at all. Phosphor lag will show up in 30fps or 60fps content, or at any framerate at that, but I think it's actually more noticeable on 30fps content (this is my opinion and I haven't done any conclusive tests regarding this). Anyways, phosphor lag is pretty easy to deal with because it really is only noticeable on bright images moving against a dark background (to me anyways), and I'd rather have a little of that than resolution loss (motion blur) and ghosting all the time regardless of framerate/source.
I would like peoples input on this topic, and it would be especially cool if some of ya'll (yeah I'm from Texas), could test this on your own displays, whether it be LCD monitor/TV, Plasma TV/monitor, CRT TV/monitor. It should be evident on any of these display devices when set to refresh at 60hz. Try a 30fps game at 60hz and look for it. Then try a 60fps game and look for it. Tell me what you find.
By the way - the most evident example I have found of this is when selecting a car in Project Gotham Racing 4 for XBox 360. As you highlight a car/bike, it drives itself onto the screen against a white background. You can clearly see the two frames being drawn almost simultaneously but with seperation as the car moves on-screen.