According to Phoronix, Canonical have started tracking Ubuntu installations. So, you are probably going to have an accurate statistics on the number in active Ubuntu installations worldwide sooner than you think.
A new package called canonical-census is uploaded to the Ubuntu Lucid repository. Package definition is something like this. "canonical-census : send "I am alive" ping to Canonical." So what the package basically does is, it sends "I am alive" ping to Canonical on a daily basis in order to track Ubuntu installations.
"The good news for those concerned about privacy is that it appears for now Canonical is just interested in tracking the users of OEM installations -- those PCs that ship with Ubuntu by default such as from ZaReason, System76, and Dell."
"This information will obviously be valuable to both companies to see whether customers are keeping around their Ubuntu installations or just wiping them and just how often Ubuntu is being used on these systems (judging by the number of times that system reported to Canonical's server previously). For those not wanting to participate in this anonymous data gathering process, they could always sudo apt-get remove canonical-census."