All you have to do is set windows 7 as the active partition, then install the update, then set Linux (OS X for me) as the active partition, reboot and you have grub (chameleon for me) ready to boot OS X | Win7 SP1 | Linux.

Use diskpart to set partitions as active. It's part of the win7 install disk.
Stick the disk in the drive, boot from cd/dvd, at the first prompt, press shift + f10 to bring up command prompt, enter:
diskpart
list disk
select disk 0
list partition
select partition 2
active
exit
exit

replace disk and partition number as needed to set windows active.
reboot
install sp1

repeat to set whatever else as active, obviously changing disk and partition to suit your needs.

Hope this helps.