apparently it's in homebrew (see other comment), but for half an eternity, I've always been using this:

    ssh user@host "mkdir .ssh && cat >> .ssh/authorized_keys" < ~/.ssh/id_rsa.pub

(remove the mkdir if .ssh already exists)

This works as well and is using nothing but the power of the unix shell and the concept of pipes.

Agreed, but that's easy enough to deal with:

`brew install ssh-copy-id`

Since it's basically just a bash-script you can just copy it from any of the major Linux-distros. I've copied mine from Debian Lenny, and it works great.