astrobruce,
If you want to do a two-way sync between your iMac and Powerbook, then, as Dave said, SuperDuper can't do it. But, if what you had in mind is a one way synce from your iMac to your Powerbook (or the other way around), then it definitely can be done from SuperDuper using disk images. I use the technique to sync my MacBook with my iMac's iPhoto library over a wireless network. (The iMac is the master.) Updates take 5 to 15 minutes depending on how much has changed on the iMac since the previous sync.
Here's the procedure.
But first, I strongly suggest you do a complete backup of your Powerbook's iTunes folder just in case anything goes wrong.
Create a SuperDuper script to copy the iMac iTunes folder to the Powerbook. (Exclude all, then add iTunes folder). Use Smart Backup and choose to use a sparse disk image - let's call it iTunes - and store it in the iTunes folder of your Powerbook. (You could put it elsewhere, but that seems like the most reasonable place.) Save the script, so you can access it again, if needed. Also save the settings as iTunes sync or any other name you like.
After you run the sync the first time, you will have a new file iTunes.sparseimage in your Powerbook's iTunes folder. Double click the file to mount the image. From iTunes preferences - Advanced, General, choose the iTunes Music folder contained within the iTunes image. With iTunes 7, you can now select from multiple iTunes libraries. See
Apple Article 304447. Following the directions, point to the existing library contained within the iTunes image.
Finally, open System Preferences - Accounts and add the iTunes.sparseimage file to the list of items to be opened when you log in.
You should now be all set. SuperDuper will smart update the iTunes.sparseimage file; at log-in the image will automatically mount and when iTunes opens it will find an exact copy of your iMac's iTunes Library and Music stored on your Powerbook. Once you are sure everything is working properly, you can delete the original iTunes libraries and iTunes Music folder.