I can't do system programming using Go on a platform for which there is no compiler. There is probably a reasonable C compiler for every platform in existence out there.
Looking at other systems such as Haiku shows that porting Go is encouraged, the only issues have to deal with assumptions in the build (in this case, I think /bin/env/bash is included everywhere, there's no global variable to change this definition from different OS types).
One day we may see a change in Go for different platforms, but I don't think it will prove to hinder that many people, only the niche groups. And for all intents and purposes, Go is built for practical application (!! Don't kill me !!), which systems programming on plan9 or haiku may not be considered "practical" at the moment.
EDIT: for clarity.
I think this is not longer a problem with the new build system that is part of Go 1.
Also AFAIK at least the Plan 9 port is almost complete, and is certainly functional.
Go is a replacement for Python, Ruby, etc. Garbage collection, duck typing, an excellent library. Go is a language with the same level of abstraction of many other "scripting" languages, but is unique because it uses static typing and is compiled.
You use the compiler toolchain in OS A, while generating executables to run on OS B.
With a bootstraping compiler, you can use this technique to bring a native compiler to new platforms.
Also, OpenBSD, NetBSD, and Plan 9 in the works.
Also there is gccgo, which AFAIK also works on Solaris and probably elsewhere.
I'd also like to add that the gc compilers support ARM as well as x86 and amd64 and are very, very easy to port to new platforms.
This is not to say Go will not get there eventually, if the language gains popularity. Someone had to write or modify a compiler to target all those architectures for C, after all. This is just a reaction to what I see as assumptions tying operating systems to certain hardware architectures (in this and other discussions). Go can be used on a range of operating systems on two hardware architectures. Very good progress, but the language needs to get that second number up before we can seriously talk about it replacing C.
[0] Only had to perform well on two, fortunately.