Heavily inspired by this blog post. With MacPorts installed:

sudo port install python easy_install
sudo easy_install -U mercurial

In your .zshrc:

export GOROOT=[path to where you want the language installed]
export GOOS=darwin
export GOARCH=386
export GOBIN=[path to where you store personal bin files]

Run:

source .zshrc
hg clone -r release https://go.googlecode.com/hg/ $GOROOT
cd $GOROOT/src
./all.bash 

That should stick some binaries into your GOBIN directory. Create a helloworld.go:

package main

import fmt "fmt"

func main() {
  fmt.Printf("HELLO WORLD");
}

Then run…

$ 8g helloworld.go
$ 8l helloworld.8
$ ./8.out
HELLO WORLD