You have to make sure you sudo it, and on occasion it has failed for me as well. Does a kextload fail as well? Make sure you are doing the AppleAIrport2.kext (not appleairport.kext)
Sometimes it fails on a first attempt but succeeds on a second, or vice versa.
I find it weird that a kextunload would fail, especially if it's run from sudo.. if you have root enabled, can you type login, login to root, and execute it there? Or does it also fail there?
the problem is that the driver, has to accept the unload attempt. and it takes the driver a while. it does not matter whether it is root or sudo.
it does help to quit all networking applications though. however kismac tries that before loading too....
can someone do some benchmarks with monitor mode disabled and with monitor mode enabled (just the wlt device created, not kismac running) it is supposed to effect the performance, however i am not sure in what dimension, and i dont have an airport extreme card to test myself. if it is not too bad, we can consider enabling it once upon kismacs installation, and disable it whenever kismac is removed...
I will run some tests later today. I suppose by performance, you mean both cpu and bandwidth?