Problem: My coworker had some issues with her wireless network at home. Her Macbook running OS X 10.5 Leopard would connect to their home network without any problems. But her husbands Powerbook and their older stationary Mac would not connect at all. There were no hardware problems with the Powerbook, in fact it worked without a hitch at her husbands job. And ofcourse they had tried retyping the password a million times.
Solution: While helping another friend of mine with his AirPort Express I came across a couple of posts in different forums that suggested using WPA2 Personal encryption (AES) instead of WPA1 Personal encryption (TKIP). We tried that and it temporarily solved the issues he had, but there were other problems in that network. However, when I told my coworker to change her encryption from WPA1 (TKIP) to WPA2 (AES) it worked like a charm and all the computers in the network got connected right away.
Short version: If you have wireless issues (inability to connect, dropping connection) on Macintosh hardware give WPA2 (AES) a try it might work.