The Raspberry Pi mini PC is a very popular platform and already a lot of developers have ported distributions for it, including RISC OS.
RISC OS is one of the oldest operating systems still around and it was initially developed by the team that built the ARM processor.
According to the developers, RISC OS is owned by Castle Technology Ltd, and maintained by RISC OS Open Ltd. This version is made available free of charge to Raspberry Pi users.
Keep in mind that it's not a final version, but a release candidate. It might have some bugs and it should be treated as a development version.
Raspberry Pi relies on an ARM processor with a clock speed of 700 MHz, 512 MB of RAM, an SD card slot and a 5V Micro USB connector that supplies the power. It also features RCA and HDMI ports.
Download RISC OS for Raspberry Pi 2012-11-01 RC6 right now from Softpedia.