Mac OS X won't boot from grub menu in Ubuntu Precise on Apple MBA(5,2)

Something like the following GRUB 2 configuration might work:

menuentry "OS X" {
    insmod hfsplus
    set root='(hd0,gpt3)'
    chainloader /System/Library/CoreServices/boot.efi
}

Note that you may need to change the partition number from "gpt3" to something else, such as "gpt2" or "gpt4". You want to identify the partition on which OS X is installed and use its partition number. To use this example, copy it to your /etc/grub.d/40_custom file in Linux and then type sudo grub-mkconfig -o /boot/grub/grub.cfg. I make no promises that this will work, though; I've not tested it.

Another option is to replace rEFIt with rEFInd and forego GRUB 2 in favor of a Linux 3.3.0 or later kernel with EFI stub loader support. Unfortunately, Ubuntu 12.04 shipped with a 3.2.0 kernel, so you'll need to replace your kernel (or wait for Ubuntu 12.10) to do this. The rEFInd documentation details how to do it (see the "Options for Booting Linux" page).

Edit your grub.cfg.

menuentry 'Mac OS X (on /dev/sda3)' --class osx --class darwin --class os $menuentry_id_option 'osprober-xnu-64-6834a4ed4dccef17'  {
insmod part_msdos
insmod hfsplus
set root='hd0,msdos3'
    multiboot /boot
}

Works fine in Ubuntu 14.04 and Mac OS X 10.9.4.