On www.macosxhints.com there was a thread about the true speed of 867MHz Powerbooks. After checking on my Powerbook I got the same result:
minime:~ christian$ sysctl hw.cpufrequency hw.cpufrequency: 533333332
The solution was a PMU reset. The whole story can be read at docs.info.apple.com, the short is here (extracted):
1. If the computer is on, turn it off.
2. Reset the power manager by simultaneously pressing and then releasing Shift-Control-Option-power on the keyboard. Do not press the fn (Function) key while using this combination of keystrokes.
3. Wait 5 seconds.
4. Press the power button to restart the computer.
(Option-Key means ALT) after doing that you may have to check if the system clock shows the correct date and time. Now you shoud see the right speed:
minime:~ christian$ sysctl hw.cpufrequency
hw.cpufrequency: 866666664