I can confirm that you can have multiple concurrent reverse ssh tunnels on the n900.

I've used this to:

1) set up a persistant reverse tunnel on 19999:localhost:22 on the phone
2) ssh into my phone from my PC on localhost:19999
3) in this shell, start a reverse tunnel #2 on 9999:localhost:5900
4) start a second ssh session on PC to 19999, then start x11vnc server within it
5) on PC, tightvnc into phone on localhost:9999

It all works OK (other than needing the phone's screen to be left permanently on for the vnc session, and failing to route some keyboard input to the foreground app) - so you can remote control the phone if you've left it somewhere inconvenient (eg. to access SMS/email/contacts/IM's etc).