It turns out that part of the 10.4.6 update was the inclusion of non-destructive partitioning in diskutil, via the new resizeVolume option.
You can use this feature, for example, to create a Boot Camp setup with more than two partitions. For an explanation, see this entry on bbum's web site.
[robg adds: This will only work on GPT (GUID Partition Table) disks, which are what you'll find in an Intel Mac. I used Disk Utility to format a FireWire drive with GPT, then used diskutil to try to resize it on the fly. I think this should work, but I was having issues with the sizing of the partitions, resulting in an error. Given more time, it should be possible. Disk Utility, however, does not seem to support the non-destructive option, even on GPT disks.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006041104380285