That number is expected to grow with their upcoming Ubuntu 8.10 release, with Canonical believing it will help their desktop side in particular. The continued development of Ubuntu is made possible by the hundreds of developers, both paid and volunteer, who maintain it. Eight million users is an impressive number for how short, relatively, Ubuntu has been around. It also retains their position as the number one choice for Linux distributions worldwide, though we all know that can change.
Depending on what estimate you use, the number of users could represent upwards of 25% of the entire Linux user base worldwide, though accurate statistics for Linux adoption are difficult to come by.