If I'm not mistaken, doesn't USB have a way for devices to access the host's memory via DMA? If so, does that mean it's possible for a 'hacked' keyboard to use DMA to write an exploit into the host machine's memory?
Why would you want to use PS3s for a homebrew supercomputing cluster if it means you have to write and optimize code for the SPEs to get benefit out of it? The PS3's linux environment doesn't let you utilize the GPU or all of the built-in SPEs and it doesn't have a lot of RAM available either. It seems like it would be cheaper to build a cluster out of commodity PC parts, and maybe use GPUs+CUDA to get more muscle without having to completely hand-roll your own accelerated computation code (since CUDA is roughly C). I can't imagine that the PS3 would end up cheaper for these purposes, considering it includes a Blu-Ray player along with a bunch of other things you're not going to be using.