I'm pretty sure they've actually made a client that can mine on it, but it gets a ludicrously low rate.
I heard this too. It sucks because the PI can't put out that much power. It's completely passively cooled, so anything more powerful, and it would just about catch on fire. I already feel bad for running my CPU/GPU on my PI at 100% for an hour or two while working or watching videos. I can't imagine the board would last long running 24/7...