Under "shared" in the FINDER, I can see the Powerbook and connect to it (ie login) and access my old files there!
If you are doing that over the wifi link (not cabled), then you have already demonstrated that it's working!
Is it possible that you have set the wifi NIC on the MBP to access Ad-hoc (Computer-Computer) networks ONLY?
Otherwise, have you set up MAC-address filtering on the wifi access point?
Are you sure that the MBP is trying to connect to your AP, and not some other one?
Another possibility is that you have set up security (WPA or WEP) on the access point. Or possibly told the MBP to use it when not required.
Given that you have no security on the wifi network, could someone else have changed anything?
SD