After some time decide to upgrade to Mac OS X 10.5 and at the same time make the partition bigger (e.g. by merging with a FAT32 partition which was originally intended to contain some Windows)
Use the GNU Parted utility to delete the Mac OS X and FAT 32 partitions and create a new bigger partition for Mac OS X 10.5.
Restart your Mac.
Cheer that now you can no longer boot from the harddisk or a CD/DVD nor from anything else!
And now, how to un-brick it again:
Call Apple support
Find out that you are lucky and have some warranty left (6 days in my case!!!).