Dual 3-tier Architecture

XML is more than just a standard medium for data communications.  It is the enabler of a new layer which I've used to create a Dual 3-tier Architecture.

What is a 3-tier Architecture?
A strategically designed database application is usually based on the standard 3-tier architecture.  The 3 tiers are: Data Backend, Application Logic, and User Interface.  For example, a typical web database application could be architected as follows:

Data Backend Tier: mySQL Database
Application Logic Tier: PHP Application
User Interface Tier: HTML output (to client)  +  GET/POST data (from client)

The important aspect of a 3-tier system is that each layer only does what it is supposed to do, and no more.  The database stores data.  The application manipulates data.  The user interface displays it in a way that is useful to the user.  Using this architecture, the database schema would not be optimized for user interface formatting purposes, but instead would be optimized to store the data in the most logical format given the read/write requirements of the application.

What is a Dual 3-tier Architecture?
Dual 3-tier includes 2 independent systems, each with a unique 3-tier architecture.  In this Flash CMS project, we will use

System 1 exists entirely on the web server:
Data Backend Tier: mySQL Database
Application Logic Tier: PHP Code
User Interface Tier: XML code (to/from client)

System 2 exisits entirely on the client side:
Data Backend Tier: XML code (to/from server)
Application Logic Tier: ActionScript Code (ActionScript 2.0 for Flash 7)
User Interface Tier:Flash MovieClip objects