A recent cnet article suggests that Ubuntu is eating other distribution's lunch. In particular, one distribution is reported to be falling apart: Gentoo. Gentoo was very popular among my friends at the time I adopted Linux, but from what I've seen, the project fell apart as developers were unable to come to consensus or resolve conflicts.
From what little I know of Gentoo users and the project, it's closer to say that Gentoo is becoming an unofficial set of distributed overlays than a centralized approved project with trusted developers and so on. Gentoo's core appeal isn't under attack by Ubuntu -- building from source and customization for performance are central and remain relatively unique. If Ubuntu's focus on desktop usability or six month release cycle are appealing enough to Gentoo users that they leave the project, then build from source and customization were simply means to an end and Ubuntu has improved the Linux landscape for the better.
The article cites Google Trends as evidence of Ubuntu rocketing off to outer space while Gentoo stagnates. I'm reminded of the current 5-a-day discussion; there's a certain amount of danger to blindly trusting metrics. This Google Trend shows "Ubuntu" approaching "Dell":
(Orange = Dell; Red = Ubuntu; Blue = Gentoo)
It's a large leap to say that Ubuntu is as popular as Dell; certainly Ubuntu is a small fraction of Dell sales. One thing I do know is that Ubuntu is very googleable. All the mailing lists are archived, IRC is publicly logged, the forums even have a special search engine mode for faster indexing and engine retrieval, bug pages aren't blocked in robots.txt, and the wiki is used extensively. This wide array of information is something a tool like Google Search can aid in, and is far simpler than say searching the wiki individually, then Launchpad and so on. Perhaps Ubuntu users are heavier users of Google Search, compensating for what could be a smaller user base. We can't really infer user base size from Trends, just growth patterns.
What does seem clear from the trend is that interest in Ubuntu isn't growing as quickly today. Every new release causes a bump in search volume but there isn't as much sticking around after the release. Maybe Ubuntu works better today, so people aren't Googling their problems as often? I am, shall we say, open to alternative explanations.