Rumours are rife throughout the internet after Nikon mistakenly posted a .pdf format of a user manual for a D50 model on one of it's websites last week. It was quickly removed, but not quickly enough it seems.
Various sites are now claiming that Nikon Japan have confirmed that the model is real, although no specs have been confirmed other than it is an entry level camera designed to follow on with the success of the D70. Futher speculation is that a D70s is on the way also, upgrading the existing D70.
The D50 certainly make sense, as it is only a few short weeks ago that Nikon registered 'D50' as a trade mark.