I've been setting up multi-boot (OpenBSD/OSX/Kubuntu) for someone else's Intel MacMini. The place where I needed to pay extra attention was making sure that OpenBSD ended up in a primary partition. That seems a bit difficult to ensure with OS X's diskutility program (which on 10.5 gives you one shot only, once you install the system the partitions cannot be changed - at least with the version of OS X I have).
I'm not entirely familiar with GPT v MBR. Nor with ReFit. Dual-boot OSX / OpenBSD seems to work just fine with ReFit. So far, when going for triple-boot, I need to chain load OpenBSD via grub like this: title OpenBSD root (hd0,2) makeactive chainloader +1 Regards -Lars