$ cat /home/tim/.ssh/id_rsa.pub | ssh tim@just.some.other.server 'cat >> .ssh/authorized_keys'
Password:
cat: >>: No such file or directory
cat: .ssh/authorized_keys: No such file or directory

$ ssh-copy-id tim@just.some.other.server

scp ~/.ssh/id_rsa.pub user@remote.example.com:/tmp/id_rsa.pub
ssh user@remote.example.com
cat /tmp/id_rsa.pub >> ~/.ssh/authorized_keys"

ssh tim@just.some.other.server sh -c "'cat >> .ssh/authorized_keys'" < /home/tim/.ssh/id_rsa.pub