The default config for airport on an ibook is to "join the last network" and to "allow this computer to create networks" and to attempt to configure using DHCP.
From this starting point, the sequence would be something like:
1. DCHP will fail to locate a DHCP server, so they will
each assign themselves an address in the 169.254.x.x/16 range, putting them on the same IP network. This has been happening since MacOS 8.5, so it's not new.
2. One (or both?) of them would need to choose the other's Airport network. I've not played with Ad-hoc wireless on Macosx, so I'm unclear on exactly what's required here.
3. One or the other of the users can then connect to a shared volume using "afp://server.local" where "server" is the rendezvous name configured in sharing. This works using
multicast dns.
There is also great potential for more than simply mounting volumes. Apple has already announced that iTunes 4 will be able to discover other iTunes libraries over Rendezvous, and Safari can discover webservers that respond to service discovery.
For itunes, we'll just have to wait, but you can get the built-in Apache server on MacOSX to show up by adding an Apache module from
Eric Seidel's site. Works really well, but is only (currently) for MacOSX (10.2.3) webservers.
Again,
http://www.zeroconf.org has a weath of information about all this...
Hope this helps..