Well, Bungie being a pretty prestigious place, I imagine that their hiring policies are fairly stringent. So regular employees are probably pretty secure if they don't -blam!- up too badly. Bungie also hires contract employees, and it's made clear to them that they're working for a certain amount of time. Contract employees can become full-time Bungie employees, but I imagine that's more likely the exception than the norm. The people who are let go after a project are the kind who should be aware that that's probably what's going to happen.