> What about other issues like posting spam and organising DDos attack?
users don't (aren't supposed to) have access directly to the compute nodes of the clusters on a particular "grid", only to the head nodes where they submit their jobs. in theory they should submit jobs from their local workstation even. ok, you can take the head node off (which may or may not be redundant) and that's it, jobs still run. technically all the main services such as schedulers, file and monitoring run somewhere else. a real malicious attack would target the main service providers such as the file server or the scheduler, without which a cluster isn't of much use. you can only do that once anyway, especially on high-profile networks. once you get caught you're off. and all those nodes are used to downtime anyways :) on the other hand it's much more interesting to try and figure out a way to usurp the schedulers so you consume more than your dedicated fair share of resources. now that's really tricky to prevent :)
