Let it sit a while longer
Let it sit a while longer, there is no harm in trying to wait it out. Make sure its plugged in so that the battery doesn't run out. It may not fix the problem, but it can not do any harm.
Is there anything else plugged in to your iBook? My wife had a Tiger update freeze because a flash drive was plugged in (I assume that it was the flash drive as that was the only difference between other successful updates).
If waiting it out doesn't work, and I mean give it at least 1/2 hour more.... then all you can is push and hold the power button until the system turns off. Wait a minute for all those racing around electrons to settle down, and then turn it back on. With just a little bit of luck, it will pick up right where it left off. Again, give it time to work through all the scripts. There is no harm that can be waiting longer than you have to. There are things you can break by being impatient. Also note, assuming that all is all on the restart, that others have reported that the system runs slow immediately after the update. This will go away after 15 to 30 minutes of use, they say, as it is the system rebuilding caches and whatnot. This is normal.
If this doesn't fix it, then I'm not sure what the next step is. (knocks wood) I've not really had to do much "fixing" with our Apples. To fix my wife's Tiger issue I updated her to Leopard.
I think your next option is to try to boot up in single user mode. But others will have to guide you here.
Good Luck
