In Jaguar and previous OS X systems, Disk Utility had a checkbox to "Install OS 9 Driver" when formatting a new hard drive. It was a tricky one -- because it only was checkable when you clicked on the HARD DRIVE MECHANISM name (and not the VOLUME name, where the checkbox was greyed.)
If you formatted the drive without this option checked, you'd never be able to use that drive on an OS 9 system... particularly inconvenient on an external hard drive that might connect to many Macs as your travel. I have a client who didn't check this box when formatting a new OS X Server on an older G4 that *used* to be capable of booting into OS 9 (which often proved helpful when troubleshooting) -- and now they can't. The internal hard drive has no OS 9 driver. In order to fix this, they'd have to back everything up and reformat -- this time being sure to check that checkbox.
In Panther's new Disk Utility, this checkbox isn't there. I formatted an external FireWire drive with it, and was still able to use the drive on an OS 9 System. So presumably the backwards-compatibility of an OS 9 driver is now included by default.

