I think this problem has always existed with the IPS panel displays, but I've never been aware of it on a brand new display before now. I noticed it on my old 23-inch Cinema Display after it was a couple years old.
On the first day, it was a big problem. The ghosting was so severe I could still read the content of closed windows on my desktop picture. Now, I can only see the ghosting on dark grey screens. This is very noticeable in iPhoto and iMovie. When I use either in full-screen mode, I can see the outline of my browser window from yesterday -- I know it's from yesterday because I made my browser window narrower yesterday and I can still see the wider outline -- and many of my dock icons can be seen. Some of the dock icons even retain the blue color, like the iTunes, App Store, and Safari icons.
So, for most use, it isn't a problem, but in some cases it is a problem, and it is very annoying. On the first day, the ghosting could be seen in anything on the screen, including color images and video, but it isn't that bad now. I haven't been able to replicate that so I don't know if it was caused by something I was doing at the time that might cause it to happen again.