On OSX 10.4.4:
System Preferences--->Displays--->Arrangement (tab).
You should see a cartoon of two displays side by side, each representing a monitor (your PowerBook's display and your attached Cinema display). One of them will have a white rectangle atop it representing the Menu Bar. Click on this little rectangle; a red box will appear around that monitor's Cartoon representation; drag the rectangle to whichever monitor you want it to appear on, and release. Presto.
I hope I have understood what you are trying to do. It may be different for a laptop display than on my two-monitor desktop display setup.