demo of this code: http://dl.dropbox.com/u/715596/autocomplete.html

address-autocomplete.html

<style>
  #q,#shadow_q {
   position:absolute;
   top:5px;
   left:5px;
   width:500px;
   height:30px;
   font-size:25px;
  }
  #q {
    z-index:999;
    color:#000;
    opacity:.75;
  }
  #shadow_q {
    z-index:0;
    color:#000;
  }
  #submit-btn {
    position:absolute;
    top:5px;
    left:550px;
    height:30px;
  }
</style>

<form id="query">
<input id="q" type="text"></input>
<input id="submit-btn" type="submit" value="submit" />
<!-- 
important that shadow_q be the last "tabbable" element, 
because the js skips focus back to first input
-->
<input id="shadow_q" type="text"></input>
</form>
<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false"></script>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js"></script>
<script type="text/javascript">
var addrs;
var accepted_suggestion = false;
var field_is_empty = function(id) {
  return ($("#" + id).val() === "") ? true : false;
};

var accept_suggestion = function() {
  return ( accepted_suggestion == false && !field_is_empty("q") && !field_is_empty("shadow_q") ) ? true : false;
};

var load_stored_addrs = function() {
  return window.localStorage['addrs'] ? JSON.parse(window.localStorage['addrs']) : []
};

var add_addr_to_stor = function(addr) {
  addrs = [];
  if (window.localStorage['addrs']) {
    addrs = JSON.parse(window.localStorage['addrs'])
  }
  addrs.push(addr);
  window.localStorage['addrs'] = JSON.stringify(addrs);
};


var geocoder = new google.maps.Geocoder();

var cur_google_suggestion = '';

var get_google_suggestions = function(addr) {
  if(addr == "") {
    cur_google_suggestion = '';
    return;
  }
  geocoder.geocode({'address': addr}, function(results, status){
    if (status == google.maps.GeocoderStatus.OK) {
      console.log(results[0].formatted_address)
      cur_google_suggestion = results[0].formatted_address;
    } else {
      console.log("uh oh");
    }
  });
}


$(function() {
  $("#q").keyup(function() {
    
    get_google_suggestions($("#q").val());

    $("#shadow_q").val(cur_google_suggestion);        
  });
  
  // accept suggestions on tab and right arrow
  $("#q").keydown(function(e) {
    if ( (e.keyCode === 9 || e.keyCode === 39) && accept_suggestion() ) {
      e.preventDefault();
      accepted_suggestion = true;
      $("#q").val($("#shadow_q").val());
    } else {
      accepted_suggestion = false;
    }
  });
  
  // if you focus on #shadow_q, move
  // focus back to #q
  $("#shadow_q").focus(function() {
    $("#q").focus();
  });
  
  // store
  $("#query").submit(function(e) {
    e.preventDefault();
    addr = $("#q").val();
    add_addr_to_stor(addr);
    $("#q").val('')
  });
  
});
</script>

Sweet!