Warning: During installation, there is an advanced option (Ready to install -> Advanced) to install the GRUB2 bootloader into the same partition into which the (K)Ubuntu OS is installed but not to change the MBR (Master Boot Record). Pay careful attention during this step if your system uses a boot partition, uses multiple OS (more than 2), or chainloads bootloaders. For systems with such a boot partition, it is best not to overwrite the MBR.

 
* Example, from the Desktop version GUI installer, a point in the installation will be reached: 
        Summary -> Advanced -> Device for boot loader installation: /dev/sda6
Mac Os on Ubuntu
In this example, this setting will cause the GRUB2 bootloader to be installed into /dev/sda6 only (the partition into which the new (K)Ubuntu OS is being installed). The MBR (Master Boot Record) will not be changed. However, if the default setting of /dev/sda is allowed, then GRUB2 will not only be installed into partition dev/sda6 (into which the (K)Ubuntu OS is installed) but also the MBR (MasterBootRecord) will be changed so that the copy of GRUB2 stored there will be designated as the master bootloader for all Operating Systems on the entire computer. This may be undesirable if you wish to use bootloaders other than GRUB2.

If you want to install more than 2 operating systems on a single computer, check out these tips.

Use Startup Manager to change Grub settings
Grub is the bootup utility that controls which OS to load by default and other bootup settings. You can change Grub settings from the Startup Manager:

System -> Administration -> Startup Manager

If Startup Manager is not installed, install it:
sudo apt-get install startupmanager

Note: You can also edit the Grub settings manually from the command-line interface.
   
Dual-Booting Mac OS X and Ubuntu
Mac OS X has a similar structure to Linux (it is BSD Unix based). Dual-booting Mac OS X and Ubuntu detailed instructions can be found here.

Installing Mac OS X after Ubuntu
if you decide to dual boot with OS X, choose ext2 as your partition type during the Ubuntu installation. (For this the Super Grub Disk CD is a useful utility. You can download the Super Grub .iso image file at forjamari.linex.org and burn the image to a CD-ROM.) 
 
* Once you have installed Ubuntu, edit the Grub start-up list:
sudo nano /boot/grub/menu.lst
    and add the following lines:
title Mac OS X
root (hd0,0)
makeactive
chainloader +1

Reboot your Mac and go to the terminal in Max OS X (if you have any issues booting, boot from your Mac OS X DVD). Press F8 and enter -s. Enter:
fdisk -e /dev/rdisk0
flag 2 <--note that flag 2 is my Mac partition number two
quit
y
reboot
If are still unsure whether it is working correctly, use the Super Grub Disk CD and make grub active. 
 
Installing Ubuntu after Mac OS X
If you get an error message during boot such as HFS+error in the bootloader, you can also use the Super Grub Disk for recovering Linux GRUB and the Windows MBR (Master Boot Record). 
 

    * Once you have installed Ubuntu, edit the Grub start-up list:
sudo nano /boot/grub/menu.lst
    and add the following lines:
title Mac OS X
root (hd0,0)
makeactive
chainloader +1

    If you have issues with Mac OSX or Windows in GRUB, try changing the Mac OS X Grub entry
    change root (hd0,0) to root (hd0,1)

This means you will boot into partition number 1. You can try any partition number until you get it right.  

Article taken from : http://ubuntuguide.org/wiki/Ubuntu:Maverick#Dual-Booting_Mac_OS_X_and_Ubuntu