Had this idea a few years back with a business model that paid publishers for cpu cycles gathered from a javascript or flash widget. We hoped to then sell this service to data-intensive industries. Decided it wasn't feasible.
We need to consider cpu cycles gained from this regime vs. bandwidth and cpu cycles lost from the hundreds of web, queue, data servers needed to run this model. IMO it is unlikely that this model will pay off once you consider things like network latency, and trade-offs like job size (higher job size is better) vs. job completion probability (lower job size is better).
Even if the potential for viability were there, it isn't clear that there is a market for something like this. Large scale computing challenges obviously exist, and a lot of people are making money with solutions like cloud computing, but these problems typically involve proprietary data sets, using proprietary or industry standard (good 'ol apps like MySQL) software. Chopping up your sensitive data and sending it en masse to the public to be processed on javascript instead of C++ doesn't exactly fit client needs.