This is a misleading statement. It could suggest computer-savy users, that N900 apps are running all the time eating CPU cycles and battery power.
When in fact Maemo (Linux) is a modern, event-driven operating system, where most of the apps most of the time are 'sleeping' and doing nothing, waiting for some event (user action, data packet on the wire, etc.) to happen. Processor is able to lower its MHz clock and even stop completely if nothing is really happening. It just sits idle waiting for an interrupt signal to wake it.

Of course, when you run CPU or I/O intensive app, the events are coming so quick, that the processor scales its MHz to the full, and app is working all the time. This is a case for games and interactive apps.

But most of the time (even if you are listening to music in the background) the machine is just idling, doing nothing.