After he posted another preview of the modifications to Blender's user interface in Fluid Designer, developer Andrew Peel received a number of requests for a more in-depth explanation of his work. In this video, he's giving a good intro into modifying Blender's UI. In addition, you can now download his scripts from GitHub.
I must say I'm astonished to see this level of flexibility using Python scripts in Blender - I didn't know you had this kind of control. Did you?
Fluid Designer Scripts
This repository holds all of the scripts needed for Fluid Designer.
Fluid Designer is developed as an add-on to Blender (www.blender.org) its purpose is to create an efficient, streamlined way to design interior environments, and create and maintain a library of assets.
To install these scripts you just merge the scripts directory into the scripts directory where you have Blender Installed. There is some functionality that requires some changes to Blenders Source Code. Full Installs for Fluid Designer with Sample Data and the necessary Blender modifications will be available soon.