Samsung Galaxy 5 Wi-fi stops working: "ar6000 driver fails to load"
I've seen a number of people report that the WLAN test dialer code will allow the driver to reload properly (example). I believe it's *#*#526#*#* on many Samsung phones, but this post indicates that it may also be *#*#528#*#* or *#*#232339#*#*. This may not work on all devices, since manufacturers tend to include/remove different dialer codes almost at a whim.

If you're unfamiliar with dialer codes, simply open your dial pad and enter the sequence as though you were attempting to dial the number *#*#526#*#*. Your phone should then jump to the WLAN test facility.