It's meaningful. It has a black background to suggest the lack of light, the power of simplicity and to contrast with the white of the text. It's the ad of the emptiness, of the void. It's an empty piece of paper where you draw your dreams, expectations and ideas.
Google Talk is the void where you put your words. It's the simplicity of an experimental software that started with the naked kernel and grows in front of your eyes.
* Note for the puzzled readers. This is a recent AdSense ad for Google Talk. Many people were curious about its meaning, so I thought it's a good idea to reveal it.