Here are some potential USB installer images for OpenBSD/amd64 5.5 http://www.nmedia.net/chris/install55.fs http://www.nmedia.net/chris/miniroot55.fs
The install55.fs contains full installation packages. The miniroot55.fs is a ramdisk-kernel only (for network installation or troubleshooting.) Please test either on as many amd64 machines as you can with any USB flash and any USB-CF adapters that you have. Report failures and success of each image ASAP. Test as many flash types (USB, CF-USB, old USB, new USB...) as you can. SPECIFICALLY, IF you have a boot failure, I need to see the dmesg output (and the fdisk and disklabel output from the machine if possible to boot it another way). Any error messages displayed from the boot blocks or BIOS are also essential.