The first set of Nikon D610 specifications received on Nikon Rumors are 100% identical with the current D600 model:
Please note that this does not mean that there may not be some other improvements that are not mentioned in the list above - Nikon Rumors still trying to get some additional information. The fact that the D610 body size will be the same indicates that this may really be exactly the same camera, just with improved shutter mechanism just like the SB-900->SB-910 update it mentioned few weeks ago.
In the meantime the price of the D600 continues to fall - Sammy's is currently selling it for $1,799.
Source: Nikon Rumors