You've got ssh running on the public ip?
You have got a strong root password - haven't you?
If you go for 10+ character alphanumeric passwords then these dictionary attacks won't get in - and don't let your users have ssh access, especially if you don't trust them to have secure passwords.
On the relatively few occasions this has happened to me I've looked up the owner of the originating ip using the tools at
www.dnsstuff.com and filed abuse reports - generally its a compromised server thats attacking you rather than some kid hunched over a keyboard.
Having something like this arrive in your logs makes you view your passwords a little more keenly!!
David