If you installed Ubuntu, do this:
Boot the Live CD, then navigate to "System -> Admin -> GParted Partition Editor" (in the menus). Only here can you edit partitions, because otherwise the HDD is mounted and GParted doesn't touch mounted partitions or devices.
Even '/' is a mount point.
I also believe Linux Mint, Fedora, and OpenSuSE have GParted on their Live CDs as well.
And the new Ubuntu 10.10 Installer has a basically n00b-proof partition editor built-in that in some ways resembles Mac OS X's Boot Camp Assistant.