I would expect to see some degree of posterisation (banding) with any digital display, I've still not found anything that can produce gradations as smooth as a good CRT. The better flat panels are good enough for most people to be happy with though.
I don't think you'll see any dithering on most of the newer LCDs, but there's a chance you'll see posterisation with that image. The 10-bit panels should be quite good though. (note: many manufacturers list the processing bit-depth rather than the panel)
You should definitely check out plasmas from other manufacturers if you would still prefer one over an LCD. (better viewing angles, colour reproduction etc.) I've heard that Panasonic is better than Samsung, and Pioneer is better than either in my opinion. PWM noise is noticeable even in bright scenes if you're looking for it with them, but the way they handle it is generally a lot less noticeable/annoying than what I've seen on other plasmas. (Panasonic has terrible dithering in dark areas, for example)