I need to install Ubuntu on my hackbook. I already have iDeneb 10.5.5 and XP booting via darwin. I wonder if it is possible to triple boot into all three via darwin. Or is it absolutely necessary to install GRUB? Where would I find a good guide about triple booting without using vmware or any other virtual environment.

Also when I try to resize my HFS partition using diskutil, it gives me error performing operation or something like that.

With your configuration is possible only with grub (it is very simple); with Vsta is possible in Vista bootloader with EasyBCD.

Giorgio