To make GRUB boot OSX do the following.

First grab pc_efi_v80 (google for it)
copy boot_v8 to something like /boot/efi
Update your GRUB with:
title MacOS X
rootnoverify(hd0,3)
makeactive
kernel (hd0,1)/efi

where (hd0,3) is your OSX partition (I hope your OSX is already MBR) and (hd0,1) is your linux /boot partition (or / if you don't have separate /boot. Fix the path too in this case).
Windows is more simple:
title Windows XP SP2
rootnoverify (hd0,0)
makeactive
chainloader +1

where (hd0,0) is your windows partition.