Well, you could set up a network drive on the wireless router for your external storage... but as far as wirelessly accessing a USB device like the sonica, I just don't think that's possible. It'll have to be plugged in to the iBook.
Thanks for the advice.
Shopping list now has
Wireless router
Network drive.
and I won't bother with the airport express I was going to get, since the Sonica will just do that and has to be connected anyway.
Keyspan has something that might work with your Sonica. It's called the USB Server. You would connect the server to the router and theoretically, any computer on the network can access any USB device on the server. I've seen the documentation and it mentions that Printers, Scanners, Hard Drives, Thumbdrives are supported, but maybe this Sonica will work too.
Send Keyspan an e-mail asking if the Sonica is supported and you'll be set.