I, too, would love to see support for RAW files - from Nikon, as you suggest, and also other camera manufacturers.
I think that the main hold up is that once uploaded, these photos need to be displayed in the web browser and although everyone's web browser knows how to decode JPGs, everyone's browser most definitely does not know how to decode NEFs.
I suspect that Nikon would want the Internet Explorer, Opera, Safari, Firefox, and Chrome web browser developers to pay them money to include the code to read NEFs and that is probably the reason why nothing has been done about it. Right now the Photosynth viewer is written in Silverlight, so if Silverlight could read NEFs, then that could work, but again, I suspect that Nikon would want compensation from the Silverlight team for the privilege of opening NEFs.
TIFFs, PNGs, JPEG2000s, and JPEGXRs are a few other image formats which would be most welcomed here by me, but time will tell.