This Wikipedia article tells you the share of web browsers from different sources, but clicking through the links you can see what penetrations OS's running web browsers have as well. These web sites give an accounting from their logs of what the OS's are for the people they're serving pages to.
W3counter has 1.49% running Linux and 0.25% running Android in October 2010.
Clicky gives a daily tally, which is 1.25% for Linux today, and has been hovering around 1.25% for the past weeks.
Statcounter has 0.78% running Linux since September. Not sure what they're counting as Linux or why their Linux count is so much lower than the others
Most interesting is Wikimedia, which really breaks down the statistics. They sample 1/1000 of their logs, so every hit they show can be assumed to be multiplied by about 1000. They count Linux, for which they include Android, as 2.04%. The breakdown is 0.75% Ubuntu, 0.47% Android, 0.07% SUSE, 0.06% Fedora, 0.05% Debian, and by the time it gets to Gentoo it is down to 0.02%. Red Hat, CentOS and "Linux Motor" (whatever Wikimedia means by that) comes up with the rest. There's even a breakdown of the different Ubuntu, Fedora and Android versions. Cool. It gives you a general idea of what the penetration rate is any way.