What about Bitcoin mining? I don't think at has been discussed before, but a central concept in Bitmining is machines collaborating in mining pools. The network requirements are low as each client downloads a chunk of data and work on it for some time, the same way as Seti@Home, compared to applications where clients need to exchange data near-realtime.
You are paid by the Bitcoin network itself for mining coins, and it actually seems some people are able to earn back the price of expensive ATI GPU's by using them for mining.
If the Rasberry in any way has the raw CPU power to compete with GPU's, or even better, if becomes possible to do GPGPU on the Rasberry GPU at some point, it could be really interesting to pay the cost of a Bramble by mining.
ARM benchmarks are here: https://en.bitcoin.it/wiki/Mining_hardw ... arison#ARM