I have experienced the same problem, and sometimes I could get through, but the connection was really really slow/laggish and would disconnect after a few minutes.

The problem is not in the SSH server because I can SSH to the device from itself. Also I can ping from the device but I cannot ping the device from outside. Also nmap won't find my maemo at all.

I got mine working by opening the browser on maemo side, now the connection works and stays stabile. Pinging the router/other machine didn't work.

Does the browser do something to the interfaces / routing rules?