Because installing 1.4 from source is already slow enough, the prospect of having to essentially the same thing twice on a new Pi led me to post this unofficial tarball for posterity.
It was built in /usr/local/go so that's where you have to unpack it.
The complete steps look like this:
$ sudo mv go.1.5.tar.bz2 /usr/local
$ sudo tar jxf go.1.5.tar.bz2
$ sudo chown -R root:staff /usr/local/go
$ sudo rm go.1.5.tar.bz2
Confirm it was installed correctly, like this:
$ go version
go version go1.5 linux/arm
You might also want to build and run the "hello, world" program to be sure.