I've had the same problem with F15 and F16 using the b43 wireless driver.
All is well with the wireless connection until it drops or I turn it off for some reason.
When it has stopped for whaterver reason, network manager cannot reconnect and it indicates that the device (wlan0) is no longer available in the network manager drop down.
In order to use the wireless connection again, I have to reboot.
Any ideas?