0, I like the idea but I still haven't found a single reason to own one.
The Pi's CPU is capable of around 0.2MHashes/sec at 3.75W[0] whereas even a weedy USB ASIC such as the BitForce SC 5Gh/s is capable of 5,000MHashes/sec at 30W[1]
[0]https://en.bitcoin.it/wiki/Mining_hardware_comparison#ARM [1]https://en.bitcoin.it/wiki/Mining_hardware_comparison#ASIC