Playing the numbers game 2008: number of Linux installations world wide
Merging different statistical data into one number is a tricky exercise and the result is questionable at best. Keep that in mind when you read the following information. The idea is not to get exact numbers but to get a rough idea of the dimension, nothing else.
Source #1: Fedora
One of the best statistical sources regarding Linux usage are the Fedora statistics. There the number of downloaded images as well as the number of unique IPs getting software updates is counted.
The data are difficult to interpret: no one knows if a downloaded image was only used to test the new system or to bun it onto a CD and distribute it to thousands of magazine readers or thousands of company computers. The second number is problematic because one new IP can mean a big NAT network or just a dial-in user who re-connected. So flaws everywhere, but it is a interesting coincidence that the IP numbers and the downloads are rather close.
There are also the smolt data.