How Customizable is the Craft Admin Interface?
There are some ways to customize the Craft Control Panel interface, but probably not to the extent that you are looking for.

The native way to do this would be through the use of the not-at-all documented template hooks similar to this.

You can search through the craft/app/templates folder for {% hook to get a full list of hooks available and the templates they live in.

For a bit more kludgy solution, you could use plugins like CP CSS and CP JS for injecting arbitrary CSS/JS into Control Panel requests.

Also worth noting this from the license agreement when dealing with plugins that modify Control Panel functionality:


  You may not write a plugin that breaks or tampers with any of Craft’s built-in functionality, including any restrictions related to license/edition enforcement, its core feature set, or its Premium Features.