I liked the predictable symmatry of the Vista and 7 lines on the graph.
It makes perfect sense, since upgrading from Vista to 7 is incredibly easy and painless (I've done it manually on about 30 machines now) so to watch the vista decline asymetrically match 7s growth is statistically pleasing.
As Graham pointed out, the stats were from people browsing the web, so these are OS installs that are being actively used, the number of CALs sold is irrelevent.
While I agree that seeing just home usage stats would be an interesting sideline, there seems to be a mythology propagated amongst fanbois that large corporations are somehow ham-strung into using Windows by evil licensing agreements, and that if we could only shake off these shackles of legacy compatibility we would immediately leap onto Ubuntu and OSX.
I hate to break it to you, but we sysadmins actually know what we are doing.
While I welcome the last near-decade or so of healthy competition from alternate OSs, Microsoft haven't been resting on their laurels. Ask yourself why young, dynamic and extremely profitable companies like Google, who have a vested dislike of MS, still use Windows in the vast majority of their offices?
Microsofts LDAP (active directory), SMB and Kerberos implementations are now light years ahead of anything offered in Linux or Unix flavours, meaning that if you're not using a Windows domain controller, you're actually losing a great deal of functionality, flexability, and even security.
Unix LDAP doesn't natively support computer accounts, and has no concept of domains - in a complex corporate environment where the computer needs to access secure resources without anyone logging in (automated installs, updates, secure lab machine only resources, automated RADIUS) this is essential.
On top of this, other services such as DHCP, DNS, certificate services, file server security and print management are all extremely well integrated, feature laiden, easy to set up, and a doddle to manage.
Most importantly, Mr Baggaley, let us not forget that it is us, the sysadmins that keep the world turning, not the fanbois sat at home play WoW in Wine, so I think you'll find the corporate figures considerably more important than home usage on many levels