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.
Well, in this case he speaks of ARM as being the slow option. It seems safe to assume he's coming from x86, PPC, et al.
There are corner cases like DSP and high speed FPGA's, but I'm not aware of any general-purpose embedded chips that make ARM look slow.
Of course, this is only my opinion, having spent the majority of my time with embedded devices at or below 1MHz. Those 72MHz ARM cores are speed demons in comparison!
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.