There seem to be several methods out there to get triple booting on the Retina Macbook Pro (rMBP). I have not seen this one before, but I may be wrong.

I managed to get Mac OS X, backtrack 5R3 and windows 7 all running off the same SSD by basically using bootcamp to install windows, then disk utility to create a new partition. Then installed linux to this partition and used gdisk (gptfdisk) to make it bootable.

More detailed instructions:
I first used bootcamp in mac os to make a working windows 7 partition.

Following this (using disk utility in mac os) resize the mac partition and create a FAT partition between Mac and win 7 partition.
This will make windows 7 unbootable for now (until you use gptfdisk to fix the mbr)

I then booted backtrack from USB (had some trouble creating a useable USB image, but finally after using the bootcamp installer to put an .iso onto the USB then using unetbootin to download and install Backtrack 5 R1 64bit gnome, it booted fine! I think I added nomodeset to grub to start but it is working fine without it now)

Then using the installer formatted the previously created FAT partition as EXT4 and set as /
Install bootloader to the partition and not the MBR (/sda4) in my case.

I then booted into the mac and downloaded gptfdisk
Use gdisk to created a new hybrid MBR option r then h in the menu. Select the partitions as listed in the GPT - selected partitions 4 and 5 (Linux and Windows). I set 4 as bootable and it recognized the partition types automatically. Say yes to placing EFI GPT first in partition and no when it asks about unused partition space. Then select w to write the new partition table. (you can use o to check before)

Following this holding down alt brought up the usual 3 hard drive icons (mac os, recovery and windows)
Selecting the windows partition booted to grub and from there you can select linux or windows.

It boots fine to backtrack but no wireless to start - To get initial internet access I used a rt73 USB wifi dongle and updated to backtrack R2.
I managed to get the internal wireless working thanks to instructions here
(I had to use iwconfig to connect as the supplied networking assistant didn't work)

Thanks to for info on using gdisk.

I now just need to get network assistant working and nvidia drivers...