It does the same thing my PowerBook G4 on 10.4, so I took it as a normal function - the Powerbook turns off the wifi and I have to turn it back on.

But I'll add, my "Evil Genius" (EnGenius) USB Wifi adapter on the same PowerBook has some strange behavior as well. Usually when I shut down I turn off the "Evil Genius" and turn it back on when I turn on the Powerbook. But when I forget to turn off the "Evil Genius" when I turn off the Powerbook, when I turn on the Powerbook again, I am greeted with a message that says "EnGenius needs to be reestablished in the Network Control Panel." But that message is in error as because when I turn on "Evil Genius" again, everything connects itself.