The version of Disk Utility that comes with Mac OS X 10.5 will let you resize existing partitions on a partitioned drive -- some of the time. What determines if it will let you adjust partition sizes or not?
Fragmentation would probably be the biggest problem. For a newly formatted partition, data will be added (more or less) from start to finish, however after a time fragmentation will leave blocks of data all over the disk. DiskUtil cannot resize a volume to a size less than the last block in use on the partition.
To defragment your partition, thereby moving all the data to the front of the disk, you can use a program like iDefrag. Alternatively you can simulate the process by copying all your data off to another disk, reformating the volume, then copying it back. You can use a program like SuperDuper or Carbon Copy Cloner to help with this process.
There may also be restrictions on resizing the startup volume. To avoid those, boot from the install DVD.