Does this sound scary to anyone else?
The difference in running Erlang here is simply that the browser would be joining a network of agents and spawning agents of its own, instead of joining a network of processes and spawning threads/web-workers of its own. This would allow a uniformity of process logic, such that an agent could be transparently running on a browser or on a server to complete a task, and messages could be being passed transparently across the network to facilitate this, without any of the code having to differentiate the cases.
NaCl ensures the browser sandbox environment remains in place--so, other than perhaps getting a bit hot, your computer won't be doing anything it wasn't already doing just running the Javascript the server served it.
For certain values of arbitrary; actually the code is restricted to the NaCl sandbox and the Pepper API.
(ignore my previous tower of babel whining wrt this particular application which sounds completely awesome)