It's hard to say that lack of sleep is fatal, because keeping subjects awake introduce a great deal of confounding factors.
There were some mice models that were used a few years back to say that without sleep you'd die. The study however depended on the mouse constantly trying to do an activity (exactly what eludes me), but when they failed, they would be dunked into water, thus arousing them. Long story short, putting an animal under acute stress, which you have to in order to keep them awake, causes a whole host of other problems. It's hard to study; hell, the exact purpose of sleep is always under debate. A few things like growth/hormone cycles are pretty much proven, but you wouldn't die from being acyclic. Anyhow,
this is an interesting read regardless.
You *can* stay awake for a lot longer than a month, under special circumstances of course. Common sense dictates that sleep must be pretty damned important as it is evolutionary conserved, and you are more vulnerable when you are sleeping. Must be quite a few reasons indeed. However as far as anything definitive, nothing is solid at all, no matter what anyone may say.