There have been a few articles recently trying to estimate the number of Linux users, which is apparently a challenging problem. However I have to wonder why it can't be figured out at least at the distro level by simply storing hashes of IP addresses that hit Canonical's update site, and looking at the number of unique ones each week/month.
There are going to be people using mirrors, but this is a small percent to lose to at least get something in the right magnitude, and the most popular mirrors could probably do a similar thing and contribute their numbers anyway. The only other main drawback would be multiple Ubuntu machines under the same IP, which again seems like it would only result in a slight inaccuracy. You'd also lose a small percent to users infrequently using their computers such that they aren't updated on a monthly basis, but yearly results would pull back in any of these people using their computers frequently enough to warrant counting.
Alternatively, as others have suggested as well, if Google would just release their numbers for browsers hitting google.com, we'd probably have a solid idea as well.
Are there already accurate numbers for Ubuntu and if not, am I missing something with my proposal?
UPDATE: Jef pointed out that Fedora is already doing this at http://fedoraproject.org/wiki/Statistics#Yum_Data, which is pretty awesome! That shows about 14 million unique repository connections, so making a VERY rough, not remotely scientific estimate, we could use distrowatch to estimate that Ubuntu has 1.68 times the number of users as Fedora, and get something around the order of 24 million users that have connected.