How does this compare to the specs on your current iBook adapter? The Apple adapters have the specs written on them in tiny print. The wattage is probably higher, but that is not a problem - it has more capacity.
I thought the iBooks has 45W adapters originally, and the PowerBooks had 67W adapters. A 70W adapter would work with either, so it is probably fine.
My initial thought is if the connector is correct, and the wattage is equal to or higher than the original, it should be fine.