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Nikon Rumors](http://nikonrumors.com/2009/05/28/nikon-d300s-specs.aspx) says:

I received that tip anonymously several days ago. The name Nikon D300s is not really what we all expected, but everything makes sense (at least to me):  same as the D300 plus HD movie mode, built-in mic and stereo audio input, dual SD and CF slots