jQuery.each( [ "height", "width" ], function(i,n){
  jQuery.fn[ n ] = function(h) {
    return h || h===0?
      this.css( n, isNaN(h)? h : h + "px" ):
      ( this.length ? jQuery.css( this[0], n ) : null );
  };
});

The (h || h===0) checking works right with undefined, null and NaN values. The original checking was h==undefined. Example the IE doesn't work correct with NaN value.

And if the h parameter is a string, but doesn't closed with "px" string, then isNaN checking is right.