The Linux Foundation started a new survey on Desktop/Client usage. Form the introduction: "The information from this survey will assist the Linux Foundation Desktop Linux workgroup to focus on areas of development that are important to you. The results of this survey may also be valuable to your business. Once you complete the survey, you will be able to view the current aggregated public results of the survey. "
No registration needed, you can see directly the results and it's even available in several languages.
But: "The 2007 Linux Desktop/Client Survey asks you to answer a few questions based on your company's desktop/client plans and not necessarily your personal desktop usage."