The developers of Blender NIF Scripts (who created the .nif exporter for Blender 2.4x) have said somewhere that they are holding off making an exporter for 2.5/2.6 until Blender Foundation release 'stable' version. To my knowledge, we've had a stable, yet incomplete, version of Blender for some time now. So either the NIF Scripts people are waiting for the final realease (2.6x probably), or a plugin in currently in development now.
Currently, there are two ways of getting Blender models into NIFs: 1) Just open up the file in Blender 2.4x, and export using the original pugin, or 2) Download NIFSkope, and use whatever version of Blender to export to an OBJ file which NIFSkope can read. In my experience, method 2 is better, because NIFSkope sets up the 'blocks' in the .nif file properly for BC (the NIF Scripts plugin didn't).
Hope this helps ;)