From the UI dept.:
The new style of Ubuntu is driven by the theme "Light". We've developed a comprehensive set of visual guidelines and treatments that reflect that style, and are updating key assets like the logo accordingly. The new theme takes effect in 10.04 LTS and will define our look and feel for several years.

...We're drawn to Light because it denotes both warmth and clarity, and intrigued by the idea that "light" is a good value in software. Good software is "light" in the sense that it uses your resources efficiently, runs quickly, and can easily be reshaped as needed. Ubuntu represents a break with the bloatware of proprietary operating systems and an opportunity to delight to those who use computers for work and play. More and more of our communications are powered by light, and in future, our processing power will depend on our ability to work with light, too.

Visually, light is beautiful, light is ethereal, light brings clarity and comfort.

Historical perspective: From 2004-2010, the theme in Ubuntu was "Human". Our tagline was "Linux for Human Beings" and we used a palette reflective of the full range of humanity. Our focus as a project was bringing Linux from the data center into the lives of our friends and global family.

wiki.ubuntu.com/Brand