Hi there,
what type of encryption is the network running? If its 128-bit wep hex, have you tried typing in the long hexadecimal sting instead of the "given password" into the password window that appears after you try to join the network? Also in this window when typing in the hex string change the drop-down password menu from password to 128-bit hex.
This long hexadecimal sting should be found in the router's encryption setup page, & may be called the "shared key".
Note: other options are available as well as 128-bit wep hex, just change the encryption type to suit. The link below I hope should explain things in more detail.
Try:
http://docs.info.apple.com/article.html?artnum=106424 & go about halfway down the page & look for "AirPort client to third-party base station" section.
I may be wrong or just have the wrong end of the stick but I found by doing this it allowed me to connect to my 128-bit wep hex system using my PB, also I think I remember reading another post awhile ago about this that seemed to say the same thing.
Its worth a go I guess, hope it helps. Post back if it works or not.
Rory