The Walkman NW-A850 series is an update to the NW-A840 series. The players have a 2.8" 240x400 OLED display (seems to be the same one as in the A840). The NW-A855 has 16GB, the A856 has 32GB and the A857 has 64GB.
They will be released in November 2010 in Japan. The A855 (16GB) will cost $280.
2.4" 240x400 AMOLED