The way my cable modem is set up, there are only 16 bits of the WPA key that aren't shared with the MAC address in an obvious way. Because of the configuration, it's impossible for me to change this. So anyone with this knowledge can break into my network by changing two characters -- pretty trivial.
Time Warner Wideband. I work around it by having a router behind the modem which firewalls off the rest of the network -- I treat the modem's side as completely untrusted. Not perfect, but it works.