Just noticed this with my 55VT50. Previously I had my TiVo set to only output at 1080i (and pass 1080p through), so didn't see it because the TiVo was doing the upscaling of 720p cable channels to 1080i. I don't watch many SD channels, but because I noticed that the TiVo's upscaling of SD was a bit soft, I wanted to see what the VT50 could do. So I changed the settings on my TiVo to output all channels at the resolution they were received, so the VT50 could do the upscaling. The first thing I noticed was that, at a viewing distance of about nine feet, the SD content was much sharper than with the TiVo doing the upscaling. That said, I also noticed that the channels broadcast at 720p actually now looked a bit softer compared to what they were previously. When I got up close to the screen I could see the effect the OP is referring to, but I'd instead say it's closer to the look of a cubist painting (but not as dramatic, obv.).
Similar to the OP, this effect is easier to see with static images - especially of people's faces -but I did in general find it harder to notice with some 720p channels compared to others. Also, to really see it I need to be right up next to the screen, about a foot or so away. But again, even though I need to be so close to really see the problem, at my normal viewing distance I'm still affected by it, as it makes the picture look softer. These reasons, plus the fact that some cable channels are broadcast in 1080i (meaning it would be impossible to see this problem on those channels) and that with other cable boxes it might not be straightforward or even possible to keep the box from upconverting 720p to 1080i, might explain why some owners of Panasonic plasmas notice the problem and others don't. On a side note, I confirmed that my brother-in-law's 65GT50 also has this problem.
I'm not incredibly surprised the VT50 has this problem just like the GT50, as my understanding is that there isn't a large difference between them (i.e. 24p/96, ISFccc, Infinite Black Ultra filter, Touch pad controller, design differences). I'm glad my TiVo can better upconvert 720p source material, but it is a bit disappointing that I'd have to buy an additional intermediary device to do so if that weren't the case.