application.js

var ConfirmedDeleteButton = Class.create({
  initialize:function(anchor) {
    this.anchor = anchor;
    if (this.anchor.readAttribute('alt').blank()) {
      this.confirmationText = 'Are you sure?';
    } else {
      this.confirmationText = this.anchor.readAttribute('alt');
    } 

    this.anchor.observe('click', this.deleteFormSubmit.bindAsEventListener(this));
  },
  deleteFormSubmit: function(ev) {
    ev.stop();
    if (confirm(this.confirmationText)) {
      var f = document.createElement('form');
      f.style.display = 'none';
      this.anchor.parentNode.appendChild(f);
      f.method = 'POST';
      f.action = this.anchor.href.gsub(/\/delete$/, '');
      var m = document.createElement('input');
      m.setAttribute('type', 'hidden');
      m.setAttribute('name', '_method');
      m.setAttribute('value', 'delete');
      f.appendChild(m);
      var t = document.createElement('input');
      t.setAttribute('type', 'hidden');
      t.setAttribute('name', 'authenticity_token');
      t.setAttribute('value', authenticityToken);
      f.appendChild(t);
      f.submit();
    }
  }
});

document.observe('dom:loaded',function(){
  $$('a.confirmed-delete').each(function(a) {
    new ConfirmedDeleteButton(a);
  });
});

routes.rb

map.resources :post, :member => {:delete => :get}

show.html.erb

<%= link_to 'Delete', delete_post_path(@post),
            :class => "confirmed-delete",
            :alt => 'Are you sure you want to permanently delete this post?' %>