Update (2011/02/06): you don't really need any of the commands below, you can just use the ssh-copy-id program that comes with ssh.
Here it is:
cat ~/.ssh/id_dsa.pub | ssh <a href="mailto:user@hostname">user@hostname</a> "cat - >> ~/.ssh/authorized_keys"
Apparently some people don't like this one that much. :-)
Here's an alternative:
ssh <a href="mailto:user@hostname">user@hostname</a> "echo `cat ~/.ssh/id_dsa.pub` >> ~/.ssh/authorized_keys"
It doesn't require the first cat or the pipe.