One can already from Mac OS X 10.4 resize partitions on a Mac volume via the command line diskutil resizeVolume:
YCO-BATIF-MacBookPro:~ yoc$ diskutil resizeVolume
Disk Utility Tool
Usage: diskutil resizeVolume [Mount Point|Disk Identifier|Device Node] size
Non-destructively resize a disk. You may increase or decrease its size.
When decreasing size, you may optionally supply a list of new partitions to create.
Ownership of the affected disk is required.
Valid partition sizes are in the format of .
Valid sizes are B(ytes), K(ilobytes), M(egabytes), G(igabytes), T(erabytes)
Example: 10G (10 gigabytes), 4.23T (4.23 terabytes), 5M (5 megabytes)
resizeVolume is only supported on GPT media with a Journaled HFS+ filesystem.
A size of "limits" will print the range of valid values for the current filesystem.
Example: diskutil resizeVol
