You can actually do this with the Mac OS X CD which comes with a version of the Disk Utility. It lets you select a partition and change its type.
The Disk Utility can be accessed by menu in the Installer application. However the menu item only seems to appear if you restart your machine and boot of the installer CD by holding the C key down. It doesn't appear if you use the Install program in Mac OS 9 to reboot your machine for you.
This is not documented and I only discovered it by accident. Knowing it before hand would have saved me a lot of time also.
Hopefully Apple will update the installer so that it allows the selection of Mac OS X (UFS) formatted partitions.