It's surprisingly hard to find statistics on linux distribution popularity by version number.
But wikipedia to the rescue. Their squid logs break down user-agent string statistics by OS. Here's the May 2010 report:
Linux seems to account for about 2% of all browsing. It looks like some browsers just report "Linux" as their OS, unfortunately. But among os-versioned browser strings, it looks like Ubuntu dominates linux distros by about 10:1, and almost all Ubuntu users are running either Lucid or Karmic (about 50/50).