faster for me (v0.10.21)

fast_concat.js

function faster_concat(list)
{
    var len = 0;
    var listLen = list.length;
    var offset = 0;
    if(listLen === 1) {
      return list[0]
    }

    for(var i=0;i<listLen;i++) {
        len += list[i].length
    }

    var buf = new Buffer(len)

    for(i=0;i<listLen;i++) {
        var src = list[i]
        src.copy(buf, offset)
        offset += src.length
    }
    return buf
}

I've made the experience that references are faster in small loops,
and the += op is not optimized in some older versions of V8, I think;
So maybe something like this:

function concat( list ) {

  if( list.length === 0 )
    return new Buffer( 0 )

  if( list.length === 1 )
    return list[0]

  var i, bytes = 0

  for( i = 0; i < list.length; i++ ) {
    bytes = list[i].length + bytes
  }

  var buffer = new Buffer( bytes )
  var offset = 0

  for( i = 0; i < list.length; i++ ) {
    list[i].copy( buffer, offset )
    offset = list[i].length + offset
  }

  return buffer

}