Dual-boot Linux on a Mac with original Boot Loader without uninstalling OS X
1) Resize your OSX Partition, leave the space created as freespace (i.e don't create any partition)

2) Write the iso to USB and boot it (take the most recent image from http://cdimage.ubuntu.com)

3) Create ext4 for / where the freespace was created, use sda1 (osx: disk1s1) or use another EFI if you have more then 1 drive and you want to use the second for linux only - in short: use the EFI partition as the "bootloader" partition

4) Install Ubuntu (or any other distro) - but watch for the bootloader, if you don't want to use grub (care to elaborate, why?) you need to rebless the volume in os'x to boot through the normal bootscreen where you can hold ALT to fire linux.

I don't know what you want to achieve with that, since having grub is basically the same, and you will be loading grub from that screen anyways since you need to load the kernel somehow - guess there are other options but I never used them, just went with grub.

What I mean is that after you select the other partition with the normal bootscreen by holding ALT, you will load grub - since you must load the kernel, and there is now way to load it directly from that screen other then using MBR which even makes less sense since it's a UEFI system ;-)

note: I strongly advise you to use refit - it's a very good solution and has some extra functionality that will make multi-os setup's boot better. 

If you don't like it, just rebless the volume in os x and you get rid of it. 
bless is a system command- however in the new os x there is: 

sudo systemsetup -liststartupdisks

and to setup back any Volume as the sys volume use:

sudo systemsetup -setstartupdisk /Volumes/YOUR_SYS_VOLUME_NAME

to check if it worked:

sudo systemsetup -getstartupdisk

So even if something goes FUBAR with the bootloader, it won't be any problem.

Why would you reinstall os x or anything else to boot linux on a Mac?!? Anyone doing reinstallations to boot linux is just a windows impared user who does not know how unix works, what gpt is and thinks of EFI as fancy bios.