Using ibook charger with Powerbook?

You should be fine but bear in mind that if you're doing anything intensive, you may end up using up the battery even if it's plugged in and charging since it may not provide enough juice to meet your needs. For general use though, it should be fine. I've used my mother's iBook charger on occasion without any obvious effects to my Powerbook.

It should be fine, although if the iBook transformer is at a lower voltage than the transformer you're getting replaced, your PB will take longer to charge. Not sure of the effect on the battery, but it should be fine I say.

And Applespider is right when she says that you may end up draining your battery anyway if the power consumption rate of the PB is higher than the rate at which your battery is charging up.

Of course, this is pure guessing. However, if you have ever travelled and had to (for one reason or another) charge your iPod through the "Electric Razor" power outlet at hotels and hostels, you'll notice how long it takes to charge an iPod fully, all due to it's low voltage output.