Booting from a dmg
I was pointed by Jean-Francois Roy that it is possible to boot off a dmg.
It's a simple two steps process using bootfolder:
./makebootfolder /Path/to/OperatingSystem.dmg
sudo ./blessbootfolder /Path/to/OperatingSystem.bootfolder
But beware: you better not have a space in the path to the dmg or in the dmg itself. If you happen to try this trick with a space in the path to the boot folder, then your Mac will brick. In verbose startup, you will get
Error loading kernel 'mach_kernel' (0x9)
If you can't boot anymore, download rEFIt and burn it onto a CD. Then boot on the CD by pressing the C key. You should see something looking like this (maybe without the linux and windows partitions):
Just press enter and you should be able to boot your Mac.