I just connected an external drive to an original Blueberry iBook using USB 1, took a few hours copying the 12GB of user files (yes, its hard drive had been upgraded) using Carbon Copy Cloner, and then hooked it up to a new intel Mac mini and ran Migration Assistant (MA). MA then cued me to copy stuff from the external drive, regardless of whether I had the drive hooked up via its supported FireWire or USB 2.0 connectors, and copied everything to the new Mac in 15 minutes. Unfortunately I had already made a new account on the new machine with the same name as the old account I was copying, so I had to make a temp account, run migration asst. from that and tell it to replace the duplicate account. Apparently you can't replace the account. while you're logged into it.
Just a note, if you're switching to an intel Mac, don't bother copying the Applications (OS 9), System Folder, or other Classic OS related items onto the USB drive to save you time since they won't work on an Intel Mac at all. Make sure to delete them from the Carbon Copy Cloner copy list before starting the copy.
This is easiest if you have a clean external disk to start from, but I did it from one that had 100 GB of other stuff on it already, and it worked fine. I just had to avoid the part of Migration Assistant that copies everything from the root of the disk, and then copy those root directories I needed by hand in the Finder afterwards.
Hope this can help someone!

