I decided a while back to try running a Raspberry Pi as a NAS. I had an old laptop serving files out to a hacked Apple TV (Gen2), I figured this had to be eating up a lot of power, so I decided to try a PI.
Success. (In my opinion.)
I have seen 10MB/Sec upload speeds, which is pretty close to the theoretical limit when using the SMB protocol.
If anyone is interested in a writeup, I would be happy to put together a step by step howto.