Hey, how can you hide the top menu bar on OS-X (Panther)?
I don't know how to hide the menu bar system wide, however, the active (topmost) application can hide the menu bar (I think the dock will hide too.) When you switch to another application that is showing the menu bar it will appear again.
This should work in all versions of Mac OS X using Carbon...
On a per-app basis, you can also set the LSUIPresentationMode plist key.
Apple Doc Page on PList keys
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