[Aetae]

<input onkeydown="return keyDown.call(this,event)" onchange="value = value.replace(/^\s+/,'')">
<script>
function keyDown(e){
  var position = 'selectionStart' in this ? 
    this.selectionStart :
    Math.abs(document.selection.createRange().moveStart('character', -input.value.length)); //ie<9 
  if(e.keyCode === 32 && position === 0) return false
}
</script>