The Nikkor AF-S 50mm f/1.8G entry just got removed from Nikon Imaging website. I guess this was a mistake and the lens will not be announced today.
Here is a quick MTF and lens design comparison between all 50mm Nikkor lenses:
Nikkor AF-S 50mm f/1.8G (unknown price):
Nikon AF 50mm f/1.8D ($140):
Nikon AF-S 50mm f/1.4G ($550):
Nikon AF 50mm f/1.4D ($470):
For DX users - here is the Nikon AF-S 35mm f/1.8G ($280):