Here are Steam's December Linux numbers that I recorded back in January:

  OS                         Share Delta
  Ubuntu 12.10 64 bit        0.29% +0.29%
  Ubuntu 12.04.1 LTS 64 bit  0.26% +0.26%
  Ubuntu 12.04.1 LTS         0.13% +0.13%
  Ubuntu 12.10               0.12% +0.12%
  Other                      0.71% +0.14%

Note that this was the first month that Linux use was recorded, which means that the delta for "Other" is almost certainly entirely Linux (for example, there was no "Linux Mint" option then).

Here's this month's numbers, for easy comparison:

  OS                          Share Delta
  Ubuntu 12.10 64 bit         0.71% +0.28%
  Ubuntu 12.10                0.38% +0.19%
  Ubuntu 12.04.2 LTS 64 bit   0.31% +0.31%
  Ubuntu 12.04.2 LTS          0.20% +0.20%
  Linux Mint 14 Nadia 64 bit  0.17% +0.17%
  Linux 64 bit                0.14% +0.14%
  Ubuntu 12.04.1 LTS 64 bit   0.11% -0.22%
  Other                       0.82% +0.06%

BTW, if you're a Steam user on Linux, don't neglect to fill out the hardware survey if you get selected. It really is an awesome resource for determining the capabilities of the average gaming computer. There's also a handful of strange results... is Firefox really six times more popular than Chrome on Windows? Are there really people still gaming on 640x480 monitors?