I've checked the in-built Airport from PowerBook 5.6 in the very same place and it works as expected under the same bcm43xx device, so I don't think it's a "no signal" issue. Unless the internal antenna is crippled somehow, but I find it hard to believe, as (as I've said earlier) the card just works on OSX.

Sadly, I have no more PowerBooks to swap and test the card :) I guess I'll have to do the swapping with my PowerMac G5.