Hi Martin, I don't think there is any widely-known problem with the latest PowerBooks, unless there is a problem related to Enterprise WPA. I see that recent updates to Mac OS X 10.4 have included some changes to Apple's WPA compatibility. Perhaps check what version the PowerBooks are running.
Also, you could try using some software like iStumbler <http://www.istumbler.net/>. For instance, it will show the number of networks in the vicinity, their strength, noise, carrier frequency, etc. It might highlight issues with fading signals or multiple base station frequencies.
