OK, I should be more specific - what I got working was an OpenBSD install on a GPT disk which included Windows 7 installed in MBR mode. This involves some nasty hackery with OpenBSD fdisk, and was done at the time because I thought I'd be installing OS X on the same disk.
What I haven't explicitly got working is Windows 7 in GPT mode on a GPT disk, alongside OpenBSD, because my BIOS only supports the ancient 32 bit EFI 1.1 (Intel server motherboard) To recap : Vista SP2 x64 supports GPT installs, as does Windows 7 x64 or better. OpenBSD does not understand GPT at all FreeBSD does NetBSD did/does(?) But only to the point of recognising, not installing on the disk. The worst of all worlds. When a GPT disk is created, in addition to the GPT format, it creates a 'protective MBR' where up to the first 2TB (limit of MBR) is covered by a partition of type 0xEE. This is to stop older programs overwriting bits of the GPT disk. It is possible to manipulate this protective MBR, and therefore install OpenBSD. Windows is remarkably tolerant of weird hard disk partitioning formats. OpenBSD doesn't know, or care. Linux does, however - it will see and use GPT. I run everything off Xen at the moment, which does now have an EFI BIOS supporting GPT I believe. It will take me a little time to allocate a disk image to install both 7 and OpenBSD on and test if it can be done. I'm fairly certain it can be done, but off the top of my head I can't remember if a GPT install of 7 has the same boot capabilities as MBR. In any case, there you go. A GPT disk can definitely support a hacked on MBR install of 7 and OpenBSD. GPT with GPT Windows and MBR OpenBSD I'd have to double check PK
