Thanks for the comments: my original post was unclear. My intent was/is to use brickhouse/ipfw to prevent all connections other than those secured by IPSec, so I get both a secure connection between the machines, and prevent unauthorised access to them as well. I don't have a router, or at least, not one that can terminate an IPSec link, so was looking to use the desktop iMac to both do this and act as an Airport base station to share my cable internet connection with the Powerbook via an ad hoc 802.11 link.

Another complication is that I'm not sure I can get WEP working on the air link (Cisco have an OSX driver for their expensive card, Asante do too but aren't available in Australia, and the open source drivers vary in their support for WEP for other cards (and may not permit ad hoc networking but that's another issue).

I'll look into the ssh tunnels- but thought IPSec was 'built into' Jaguar (ie with a GUI) and might be easier/faster to set up. If anyone has done this, I'd appreciate hearing about how it was done.