Thank you, Lord Anubis, for clearing things up. 
However...
depending on your os version, You can check which one exists, use
rm /System/Library/Extensions.mkext
or with 10.6
rm /System/Library/Caches/com.apple.kext.caches/Startup/Extensions.mkext
The part in bold looks a bit strange to me. Is there a part of the sentence missing? From the looks of it, the commands serve to clean up Mac OS X's kext cache.
Yes, your right, it reads strange and there should indeed a sentence be there.
Actualy, I did meant to write.
You can check which one exists.
Use
rm /System/Library/Extensions.mkext
or with 10.6
rm /System/Library/Caches/com.apple.kext.caches/Startup/Extensions.mkext
to remove the old kext cache.
But even that isn't very clear.
So I did update my former post.
Thanks for the correction.