Often enough companies are claiming, that the amount of people considering Linux for gaming is rather low. If you check the splatterladder for Enemy Territory: Quake Wars you will see in the live statistics, that the number of Linux players is just as high or higher as the number of Windows players. In this very moment the percentage of Linux players is 54%!
I don’t know how splatterladder derives their client OS data, but as of this writing the breakdown is still split in favor of Linux (57% to 43% Windows). This is rather surprising, as, if memory serves, id Software games usually have around 5% of players using the Linux clients. I wonder if this is an artifact of the server breakdown, which is a more understandable 60% Linux to 40% Windows.
At any rate, there certainly are many Linux ET:QW players, which is gratifying to see. Besides splatterladder and the official stats site, there’s the ET:QW Stat Reader, which will generate a table of performance data from your player name.
Edit: Thanks to Timbo for providing additional data that makes this look very unlikely: the open source first-person shooter Tremulous, which is included in some Linux distributions, has Linux clients making up around 16% of their total playerbase.
It’s players on linux servers.