The unthinkable is happening – Linux Mint is overtaking Ubuntu in popularity.
Watching
DistroWatch.com recently, a popular aggregator for Linux distributions and a barometer of Linux distribution popularity, Ubuntu -- the darling of desktop Linux -- has at times been no longer number one. Ubuntu is threatened by its rival, Linux Mint, which recently overtook it. (Although right now, it's back in the number one spot.)
However, measuring Linux distribution popularity is far from a science. As a freely available operating system, Linux distributions can be mirrored among thousands of independently owned sites, distributed on magazine cover CDs, sold on discs and so on. Meaning it's impossible to truly know how many people are using any particular distribution of Linux.

On the up: Linux Mint.
But as a central site for Linux distributions, DistroWatch's page hit ranking has always been a good guide of popularity, even if not actual usage.
The change in ranking seems not so much to be a result of Ubuntu losing any popularity, but that Linux Mint has gained it. Its focus on ease of use through improved applications (especially the Software Manager and Update Manager) and a good default desktop configuration out of the box has seen its popularity steadily grow over the years.
But ultimately it's a win for Ubuntu, as Linux Mint is itself based on Ubuntu. Of note however is that Linux Mint has chosen not to adopt Unity, the new interface built from the ground-up that saw its debut in the latest version of Ubuntu, 11.04 aka Natty Narwhal.
Linux Mint 11, the current version, sticks with the traditional Gnome desktop but does take advantage of other Ubuntu 11.04 features, such as overlay scrollbars.
Linux fans are far from fickle however, and the rankings on DistroWatch’s change daily. We'll have to see if Linux Mint maintains its growth, or if Ubuntu claws its way back to a definitive top dog status (or rather top penguin) again.