Granted that page doesn't distinguish between x86 and x64 but it does answer some questions. Also this Softpedia article might help to shed some light.
The 4 most used OS's
1) Windows XP - 25,000+ units
2) Windows Vista - 5,557 units
3) Windows 7 - 4,153 units
4) Linux - 2,766 units (List of The 10 Most Popular Linux Distributions)
Here is my list of Os's I currently use.
1x Windows 7 - Home Premium x64 (Core i7-920)
1x Windows XP x64 (Phenom II 820)
2x Windows XP Pro x86 (P4@2.8Ghhz-Northwood / P4@3.2Ghz-Prescott)
1x Windows 2000 Pro (P4@2.8Ghz-Prescott)
2x Ubuntu 10.04 LTS x86 (2x Pentium M@1.7Ghz)
I wish I had more team related info for you.