If you have the network settings set for Automatic, the PB will attempt to find any network access, Ethernet, wireless, modem. It is possible that the airport card could kinda see the network at work but could not access it. If you check the IP addresses you will probably find that they are similar. Having said that, the PB should have been able to access your home network without any problems what so ever. Unless you are setting your own IP address and NOT allowing DHCP to work.
Check the DHCP settings to see if you have the machine set to get its IP address from the Wireless base stations.
Hope this points you towards an answer