[Majost]

Is your username/password guessable? If so, I really wouldn't be surprised if some dictionary attack succeeded. There are a gazillion SSH dictionary attacks going on all the time.

When I had a machine open to the world with SSH, I got really tired of all the denied entries in my SSH log. So, I just changed which port was forwarded from my router. Instead of linking the router's external port 22 to the Mini's port 22, I linked some random external port, like 23422. While I must stress that this does *not* increase your security, it does reduce the number of dictionary scripts that hit your machine.

You can do much more complicated things that actually do increase your security level, but I would start with a stronger password and moving SSH off of port 22.