This may not be exactly what you're looking for, but I wanted to be able to ssh in to the n900 over gprs and my carrier seems to be blocking all incoming ports.

What I ended up doing was setup an openvpn connection to a server I have access to. Since it's an outgoing connection the carrier doesn't limit it. Then, I can use that server as an intermediate hop to ssh into the phone via the vpn.

An added bonus is that openvpn is very resilient to sporadic connection errors and will auto reconnect if gprs drops for a bit.

I'll be glad to post the config files for openvpn and detailed instructions if you'd like them. I used the openvpn package as well as the openvpn-applet to provide a statusbar control for it.