@Switch_ didn't you say you had 50% CPU while N900 is hard off? This is ridiculous, when phone is OFF nothing is running, no CPU. I assume your Battery Graph is simply showing nonsense. How would Battery Graph know what is going on in CPU when system is OFF?

If your battery drains while phone is off, either the phone hardware has a hard failure (N900 -> Nokia Service) or the battery is dead (easy to replace).