Cortex M3 doesn't have an MMU so it won't run Linux or other desktop style operating systems. Generally they are programmed 'on the bare metal' or with a fairly lightweight RTOS. So depending on what duaneb wants to do it may not be appropriate.

If the idea is just "find out how well my code runs on a slow machine" a OpenWRT box is probably a decent bet. They are often MIPS core based but when programming in a high level language that doesn't really matter.