What steps will reproduce the problem?
The debug/macho package provides utilities for reading the headers and load commands of
Mach images, but it only supports thin binaries. On OS X, it's fairly common to produce
fat binaries that contain multiple MachO images for different architectures. Examples
include (i386 and x86_64) and (armv7, armv7s). I think the debug/macho package should
provide support for reading fat binaries as well.

https://developer.apple.com/library/mac/documentation/DeveloperTools/Conceptual/MachORuntime/Reference/reference.html#//apple_ref/c/tag/fat_header

To preserve the existing API, two new types would probably be necessary:

FatFile and FatArch. FatFile would contain the magic and a slice of FatArchive.
FatArchive would include the fields from the fat_arch struct and also provide a way to
get a *macho.File object for that architecture.

What is the expected output?
N/A

What do you see instead?
N/A

Which compiler are you using (5g, 6g, 8g, gccgo)?
6g

Which operating system are you using?
OS X 10.9.1

Which version are you using?  (run 'go version')
go version go1.2 darwin/amd64

Please provide any additional information below.