ok, not really a fix. when using the ath5k module (similar probs using the madwifi mods with kernel 2.6.24), occasionally i was unable to find anything with "iwlist wlan0 scan" ("no results"). dmesg gave me
ath5k phy0: unable to reset hardware: -11
ADDRCONF(NETDEV_UP): wlan0: link is not ready
anyway, after much googling and finding the problem (but of course, not the solution) posted, it turns out that a hard power down is necessary to reset the radio. just doing a "restart" isn't enough, the computer must be physically off. hopefully this saves someone some time...