It depends on which iBook you have and which PowerBook keyboard you are looking at.
For instance, the Titanium PowerBook keyboards will work on non-opaque G3 iBooks (the ones with the clear shells painted on the inside).
The newer Aluminum PowerBook keyboards will not work as they don't have the 2 tabs on the top (between the esc and F1 and the F11 and F12 keys) to pop into the iBook.