I was able to get in using key-based authentication.
First, go to the dd-wrt web interface and enable SSHd. It is under the SERVICES tab, SECURE SHELL. Enable SSHd, and disable PASSWORD LOGIN.
Go to your command line and generate your public key:
This command generates a public key for you in ~/.ssh/id_rsa.pub. CAT the contents of that file and copy it to the dd-wrt web interface:
$bash-2.xx: cat ~/.ssh/id_rsa.pub
It will output something like this:
Copy that into the AUTHORIZED KEYS field in dd-wrt's SERVICES>SECURE SHELL
Hit apply, then reboot the router from the REBOOT ROUTER button at the bottom of the interface page.
Now log in via ssh on the command line:
$bash-2.xx: ssh root@
All of this in on the dd-wrt wiki page: External SSH