This really should have been more obvious to me, so why did I have to look it up?  If you need to check for NULL in a returned field from SQL (or any other datasource), you should the .Equals method on System.DBNull.Value.  I usually prefer the "==" notation for conditionals, but that's just me.  This is more efficient.  The code follows:

if (!dsSpecQuery.Tables[0].Rows[0]["device_type_end"].Equals(System.DBNull.Value)) 
{
    _EndDate = Convert.ToDateTime(dsSpecQuery.Tables[0].Rows[0]["device_type_end"]);
}

Of course, dsSpecQuery is a DataSet, and the field in question is DateTime out of SQLServer (not that it matters for the conditional).